Move global Meter and Tracer getter/setters into dedicated globals package

[jmacd]

The proposal is that we move all globals out of the main trace and metrics package. The global tracer and meter objects will be registered in another package, opentelemtry, along with the Init() method discussed in OTEP 0005.

Eliminating globals from the main packages allows globals package to auto-initialize by loading a plugin, for example; if we separate globals from these packages now, this can be a future option. If we leave globals in the main packages, we cannot do this. It is impossible to auto-load a plugin that depends on the package that is loading it. When there are no globals in the packages themselves, it becomes possible for a binary plugin to to be loaded that implements the Tracer or Meter interfaces, at the first moment when the globals are referenced.

There is an old proof-of-concept showing how this could work in the future, provided that we separate globals from the main API packages now. #52

[jmacd]

More concretely, I propose the top-level opentracing package contain something like these three methods:

Init(TracerFactory, MeterFactory) initializes opentracing global factories
GlobalTracer(name string) Tracer gets a named tracer from the global tracer factory
GlobalMeter(name string) Tracer gets a named meter from the global meter factory

[lizthegrey]

I 1000% endorse this.

[rghetia]

@rghetia to post the PR with named tracer.