Send to multiple exporters from tracer

[c24t]

This is an experimental branch to address #181. It changes Tracer such that it can use multiple exporters at once.

[flands]

@c24t this appears to work, thanks! note, there are other parts of the library that also need to be updated. For example, opencensus/trace/ext/flask/flask_middleware.py

[ocervell]

@c24t looks cool. Does that mean we'll have one background thread / exporter here ?

[c24t]

Does that mean we'll have one background thread / exporter here ?

We will for each exporter that's using a background thread transporter, but we're blocking between calls to export. If we expect users to use many exporters with blocking transports at once we might want to make the calls from (e.g.) an async executor instead.