allow platform-override in capabilities

This is a workaround for not being able to use any "platform" capability outside of the Platform enum  (http://selenium.googlecode.com/git/docs/api/java/org/openqa/selenium/Platform.html). 

This causes a problem, at least with Sauce Labs and Safari 6 (which requires platform = "MAC 10.8"). Using that with the previous code resulted in:
"No enum const class org.openqa.selenium.Platform.MAC 10.8  java.lang.Enum.valueOf"

Not sure this is the approach you want to take, but this was the only workaround I found. A more thorough workaround would be to allow more flexible construction of the DesiredCapabilities object by users of the lib, which I can attempt if you're open to that...