Move azure exporters into separate repo

[c24t]

Following open-telemetry/opentelemetry-specification#277 (comment), we need to move third party exporters out of the main repo. Azure is the only such exporter now, we'll have to do the same thing for stackdriver soon.

[reyang]

@victoraugustolls FYI.
@lzchen please keep Victor posted on the progress.

[lzchen]

Should the discussion of where we are moving third party exporters to take place here or in the community? It would make sense to align the decision across all libraries, so I feel like it should be a community discussion.

[carlosalberto]

@lzchen I think that most vendors will probably desire to have it in their own organization - and for those ones that don't, we should be able to put that in opentelemetry-contrib (I think @reyang will talk/ask Sergey about this).

[lzchen]

Closed with [#304]