Decide the distribution model

[rakyll]

The package is currently being distributed per aspect, e.g. pip install opencensus-trace. For other languages, our policy is to distribute trace, tags and stats in the same package. There are two reasons we are considering this option:

i. Trace, tags and stats are very thin and bundling them together is not costly.
ii. OpenCensus is a unified tracing, stats and propagation library and giving user the opportunity to install a single package is aligned with its unified nature.

Hence, I am suggesting we should distribute the package as "opencensus" with trace, stats, and tags.

[liyanhui1228]

We'll release as a single package for trace, and stats, tags in the future.

[liyanhui1228]

https://pypi.python.org/pypi/opencensus