Support of undocumented feature: :sessionId/file

Generates a zip archive locally with a given file. Returns the full path of the transferred file (unzipped) on the remote RC.

TODO: tests

Example usage:

`````
$path_of_file_on_remote = $this->prepareSession()->file( 'path/to/my/file.jpg' );
`````

This makes it possible to transfer files from the machine running a test, to an RC that's being controlled by selenium grid.