Cordova file plugin compatibility

This patch adds:
- window.requestFileSystem compatibility, has seen in Android 4.4 WebView
- cordova compatibility by waiting for the 'deviceready' event to request the file system

Should this really call the success function in the event that the method doesn't exist? Seems counter-intuitive, since if the function doesn't exist then it wasn't successful. If this is specifically for cordova support (ie, cordova never has this method available), I would suggest calling successFn only if window.cordova is true, otherwise call errorFn.