Add mocking of win32com when building docs on RTD

Read the Docs doesn't have (many) C libraries, such as win32com used by
ConnectionGrammar. Sphinx' autodoc breaks because of the failing
imports. Here I have added mocking of that package, so that the
import is successful and autodoc can run. This change is based on the
description here:
http://read-the-docs.readthedocs.org/en/latest/faq.html#how-do-i-change-behavior-for-read-the-docs