[media] Added get method to request arbitrary key

New "get" method added to GriloMedia.

Now, it is possible to request the value stored in
an arbitrary key through its name.

The key can be a standard one from Grilo core or
another one dynamically added to the registry by a
plugin or application.
Merge pull request #13 from Igalia/get-arbitrary-media-key

[media] Added get method to request arbitrary key