doc: Add documentation for VSHelper.h
doc: Small change to wording
doc: Add some plugins to the list
doc: Python 3.3 is needed after all

This was mistakenly changed in b7028566acb38cb7a421090fa565c91b8566e33d.
doc: Explain the "flags" member of VSVideoInfo
doc: Document new nfIsCache flag
Merge pull request #87 from dubhater/doc

Documentation for VSHelper.h and other things