Merge from pants repo upstream. - #17

Required moving the .proto files to be
below the exported target (in the filesystem
sense).

If we support setting a source root on
generated protos in the future, we can
modify this further to demo that.

Remaining followups:

* [ ] Fix `./pants run helloworld/main.py` failing due to import error. Test it in CI.
* [ ] Turn on dependency inference. Needs docs update.
* [ ] Enable MyPy.
* [ ] Possibly change Protobuf structure to show off the new `python_source_root` plugin field. Needs docs update.
* [ ] Use `resolve_all_constraints`. Needs docs update.
* [ ] Fix all references to readme.io. Fix the `master` branch too.
* [ ] Update the `./pants` script. Fix the `master` branch too.

Otherwise `./pants run helloworld:helloworld_py2` doesn't work.

Update the ./pants wrapper to add support for specifying a main branch version of Pants via PANTS_SHA.

This shows off the MyPy Protobuf plugin and partitioning based on interpreter constraints.

This corresponds to an updated entry for https://www.pantsbuild.org/docs/python-interpreter-compatibility#changing-the-interpreter-search-path.

This prepares us for 2.2 removing support for running with Py36.