Explicitly defined extension names

This pull request changes two things related to extension names:

- To avoid having to do stuff like `extension.name.replace('Mopidy-', '').lower()` I've renamed `Extension.name` to `Extension.dist_name` as this is the distribution name used on PyPI. Then I added `Extension.ext_name` which is the name used in `entry_points` in `setup.py` and in the config section names.

- Since we have used `mopidy.ext` everywhere else, I've changed the entry point name from `mopidy.extension` to `mopidy.ext`.