Moving active span interaction To TracerProvider

Fixes open-telemetry#455.

The previous spec stated that the active span get / set behavior MUST
behave the same across Tracers retrieved from the same TracerProvider.
As such, the active span manipulation methods are better provided by the
TracerProvider directly.

As it is still cumbersome to retrieve the current TracerProvider simply
to get the current span, add in the concept of a "Trace Package", that can
provide utilty methods to delegate to the configured default TracerProvider.
This enables a fluid interface to retrieve the current span:

    from opentelemetry import trace

    current_span = trace.get_current_span()

To help maintain backwards compatibility, the Tracer may still expose
active span methods.

specification/trace/api.md

@@ -9,6 +9,7 @@ Table of Contents
   * [Time](#time)
     * [Timestamp](#timestamp)
     * [Duration](#duration)
+* [Tracer Package](#trace-package)
 * [TracerProvider](#tracerprovider)
   * [TracerProvider operations](#tracerprovider-operations)
 * [Tracer](#tracer)
@@ -74,6 +75,18 @@ A duration is the elapsed time between two events.
 * The minimal precision is milliseconds.
 * The maximal precision is nanoseconds.
 
+## Trace package
+
+The `Trace Package` is the package containing the TracerProvider and Tracer
+API classes. In some languages, this may also be referred to as a module.
+
+The `Trace Package` MAY provide the following functions:
+
+- Get the currently active span
+- Set the currently active span
+
+Both of these operations MUST delegate to the configured `TracerProvider`.
+
 ## TracerProvider
 
 `Tracer`s can be accessed with a `TracerProvider`.
@@ -97,7 +110,7 @@ number of `TracerProvider` instances.
 
 The `TracerProvider` MUST provide functions to:
 
-- Get a `Tracer`
+#### Get a `Tracer`
 
 That API MUST accept the following parameters:
 
@@ -129,6 +142,17 @@ the tracer could, for example, do a look-up with its name+version in a map in
 the `TracerProvider`, or the `TracerProvider` could maintain a registry of all
 returned `Tracer`s and actively update their configuration if it changes.
 
+#### Get and set the currently active span
+
+The `TracerProvider` MUST delegate to the [`Context`](../context/context.md) to perform
+these tasks, i.e. the above methods MUST do the same as a single equivalent
+method of the Context management system. However, `TracerProvider` implementations
+are not required to behave the same way.
+
+For example, different `TracerProvider` implementations may use different context
+keys to get and set the currently active span. This would result in a different active
+span depending on the `TracerProvider` you are calling to.
+
 ## Tracer
 
 The tracer is responsible for creating `Span`s.
@@ -142,17 +166,12 @@ The `Tracer` MUST provide functions to:
 
 - [Create a new `Span`](#span-creation) (see the section on `Span`)
 
-The `Tracer` SHOULD provide methods to:
+The `Tracer` MAY provide functions to:
 
-- Get the currently active `Span`
-- Mark a given `Span` as active
+- Get the currently active span
+- Set the currently active span
 
-The `Tracer` MUST delegate to the [`Context`](../context/context.md) to perform
-these tasks, i.e. the above methods MUST do the same as a single equivalent
-method of the Context management system.
-In particular, this implies that the active span MUST not depend on the `Tracer`
-that it is queried from/was set to, as long as the tracers were obtained from
-the same `TracerProvider`.
+These functions MUST delegate to the `TracerProvider`.
 
 ## SpanContext