Introduce Telemetry Schemas (open-telemetry#152)

Resolves open-telemetry#1324

I believe changes to semantic conventions and the shape of emitted telemetry
are likely to occur during the lifetime of an instrumentation library.
I do not think we should aim to lock the telemetry schema and disallow changes to it.
Such locking would place a huge limitation on how instrumentation can evolve and
would make it nearly impossible to fix mistakes in the semantic conventions, in the
schema or  in the implementation of the instrumentation (which will inevitably happen
sooner or later).

This OTEP introduces the concept of telemetry schemas that allows semantic conventions
and instrumentations to evolve over time without breaking consumers of telemetry.