Get/Set formats in a distro-agnostic way

[davidmhewitt]

Fixes #76

Means we can get/set the user's format preference in a way that works on non-Ubuntu based distributions. I've tested that this has no effect on elementary but means that the plug correctly detects and allows you to change format settings on Fedora.

To test:

• Change the "Formats" property in the plug
• Log out, log in again
• Check that new formats have applied (e.g. date format in wingpanel has changed, language has remained the same)

[danirabbit]

Makes sense! Thanks for fixing this