Cross platform plugin features

Hello @sjkaliski, 

Please review the following commits I made in branch 'larzconwell-cross-platform'.

60006ff849ed30c756ec98025ff2dfc9f3c07900 (2014-08-28 16:30:52 -0400)
Include info about option OS specific hooks.

6174ecd3bd2f21f0157b15c75e46cd1c273331ba (2014-08-26 15:31:03 -0400)
Clean up README and add info about requirements object.


R=@sjkaliski
VISIBLE_CHANGES=Cross platform plugin features