Dynamically get version component of build directory for building sphinx docs

This almost certainly isn't perfect, but it's a big step up and should
work on (at least) any 32-bit or 64-bit version of python2 or python3
running on Linux (rather than just 64-bit python2.7 on Linux).
Document list of parameters for the Keypair credential callback.

Includes a couple of small grammar fixes to other docstrings.
Further clarified purpose of user field in docstring for Keypair class
Fix syntax error
Merge remote-tracking branch 'djmattyg007/doc_updates'
docs: fix reST errors
Fixed the chicked-and-egg problem with CFFI in setup.

This still needs some work in order to fix the problem
under windows. I have no windows machine here to test it.
Forgot the `setup_requires` argument.
BuildWithDLLs now inherits from CFFIBuild to *theoretically* fix windows.
Merge remote-tracking branch 'opebeat/cffi-install-fix'