Make PyPolyChord pip-installable.

[appetrosyan]

This is one of the major missing features of PC, and shouldn’t be too difficult to implement.

[appetrosyan]

The work that needs to be done is

1. to automate fetching dependencies
2. to automate fetching build dependencies, which should be similar to how jumpy does it (they have fortran code too).
3. test the reproducibility if the build on Windows, Linux and Mac OS X.
4. add optional dependencies,
5. handle MPI
6. expose 3, 4, 5 to pip in a standard way.

[williamjameshandley]

I believe that we were nearly here with #24. It looks like the only thing blocking this were OSX related issues. I suggest we move this conversation to there.