Add File plugin

For now, it's only a wrapper of requestFileSystem : http://cordova.apache.org/docs/en/3.3.0/cordova_file_file.md.html#requestFileSystem

For the FileEntry, DirectoryEntry, FileTransfer, etc. they are object oriented. So a simple wrapper of methods isn't enough. Adding every time the object to a method is a pain in the ass.

We need to wrap the all object. What do you think ?