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.

…eraction-from-contextmanager

Removing the get / set current span methods from the TracerProvider.

There is consensus that the functions should have the same behavior
regardless of the TracerProvider.

This change now means that the span retrieval methods can not be defined
completely by the Trace Package. Moving the requirements up to the package level.

Selecting "Tracing Context Utilities" rather than "Trace Package" to provide
clear guidance on languages that do not have static module-level functions.

Adding a section on the ability to expose these as module-level functions to ensure
that the simplified workflow of a top-level get / set span is possible.

…nager

…nager

…nager