Add a script to generate a PyCharm-compatible virtualenv.

Note there are a few changes to test code to allow running without pants as the test runner:

* Use relative imports in test packages.
* Added blank `__init__.py` files to test directories that need to use relative imports now.
* Catch PexInfo errors that result from not running inside a PEX.
* Add requirements.txt to .gitignore (we generate it from pants) - it helps
pycharm offer quick fixes and identify packages with updates available.

To create a PyCharm project:

Set the project interpreter to `build-support/python/pycharm.venv/bin/python`.
Mark `src/main/python` as a Sources Root.
Go to Settings > Python Integrated Tools and set `Default test runner` to `py.test`.

Testing Done:
./pants src/test/python:all

./build-support/python/make-pycharm-virtualenv
source build-support/python/pycharm.venv/bin/activate
PYTHONPATH=src/main/python py.test src/test/python

Tested PyCharm Community Edition 3.4. Basic navigation (Ctrl-click) works. Find usages
works. Py.Test works if configured manually. Not a great experience, but better than no IDE
whatsoever for some things.

Reviewed at https://reviews.apache.org/r/27364/