Save theme directory instead of theme name in mdm.conf.

This fixes the loading errors in the cases when the theme's dir
isn't the same as its name (see Brit Waves, Linux Mint themes)
or, even worse, the theme name is localized while its dir isn't,
obviously (see Circles theme).

Fixes https://bugs.launchpad.net/bugs/1392787
Merge pull request #127 from monsta/names

Save theme directory instead of theme name in mdm.conf.