Automatically create and activate the virtualenv of the test corpus

[eddieantonio]

In order for UnnaturalCode to run files from the test corpus, UnnaturalCode must activate its virtualenv, if any.

It can infer to create a virtualenv. If the project includes a file called requirements.txt:

1. it should create a virutalenv
2. it should invoke pip install -r requirements.txt inside the project's virtualenv
3. it should activate the virtualenv using activate_this.py

[eddieantonio]

P.S., this is why the build is broken as of 09f517d 😬

[hazelybell]

That's what the VIRTUALENV_ACTIVATE environment variable is for. 😏