Adding Context API (#395)

This change implements the Context API portion of OTEP #66. The
CorrelationContext API and Propagation API changes will come in future PRs.
We're leveraging entrypoints to support other implementations of the Context
API if/when necessary. For backwards compatibility, this change uses
aiocontextvars for Python versions older than 3.7.

Co-authored-by: Diego Hurtado <ocelotl@users.noreply.github.com>
Co-authored-by: Mauricio Vásquez <mauricio@kinvolk.io>

tox.ini

@@ -82,6 +82,7 @@ commands_pre =
   jaeger: pip install {toxinidir}/opentelemetry-sdk
   jaeger: pip install {toxinidir}/ext/opentelemetry-ext-jaeger
   opentracing-shim: pip install {toxinidir}/opentelemetry-sdk {toxinidir}/ext/opentelemetry-ext-opentracing-shim
+  zipkin: pip install {toxinidir}/opentelemetry-sdk
   zipkin: pip install {toxinidir}/ext/opentelemetry-ext-zipkin
 
 ; In order to get a healthy coverage report,