Store 2 internal copies of firefox and updated selenium fixture for using them.

Setting the browser to firefox24 will use the internally stored Firefox
v24.8.1
Setting the browser to firefox31 will use the internally stored Firefox
31.2.0
Setting the browser to just firefox will use the system's installed copy
of firefox
The internal copies are located under Oasis\apps\ and are extracted when
running ant-run.bat
Better name for new selenium method loadFirefoxFrom