Add Timeout option to FirefoxProfile to enable custom browser launch timeout

Hi @jimevans, 

As reported at [#8202](https://code.google.com/p/selenium/issues/detail?id=8202), since the upgrade to Firefox 32 I'd started seeing some tests failing due a reach of hardcoded 45 seconds timeout to Firefox become ready to work.

This pull request just add a Timeout to FirefoxProfile in order to make it available to set from regular RemoteWebDriver's DesideredCapabilities.

Cheers,
Gustavo