*Added package 'multirows' to included documents in pylatex/Document class.
*Added 'add_multicol' and 'add_multirow' functions to table class.

(This is my first modification to PyLaTeX, I don't know if I followed the
 standards or added the 'multirows' package in the correct place.)

This commit is intended to fix issue #4: "Add multicol/multirow support for tables."