Adds opt-in support for loading the SDK's configuration from the shared
config file (~/.aws/config) in addition to the current shared configuration
file (~/.aws/credentials). This option can be enabled with the env var
`AWS_SDK_LOAD_CONFIG` set to a truthy value, or creating a session with the
`session.NewFromSharedConfig()` helper instead of `session.New()`.

In addition to loading from the shared config file the SDK will also now
load region from the shared credentials file when the share config is
enabled. This changes also treats the syntax of both shared config and
credentials as the same.

See the session package for more information and examples.

Adds shared config support for aws#384

Updates the SDK to take advantage of the new `AWS_SDK_LOAD_CONFIG`
environment variable. When set to a truthy value, as defined in
strconv.ParseBool, the SDK will load both shared config file
(~/.aws/config), and the shared credentials file (~/.aws/credentials).
The shared credentials file has precedence over the shared config.

Both config files share the same format. Both `[name]` and `[profile name]`
are accepted as section names. The first, `[name]` will be looked for
first before falling back to `[profile name]` per config file.

See the [session](http://docs.aws.amazon.com/sdk-for-go/api/aws/session/) package docs for more information.

Deprecates the `session.New` method since it cannot return an error
when the session fails to load until a request is made.
`session.NewSession` corrects this by returning an error when creating
the session.