Added update checker class.

Uses GitHub releases feature to detect whether a new release is available.
Added variant property to Updater.

Used for platform specific releases. E.g., 0.3.2.1-sailfishos will only be
reported as new release over 0.3.2 if variant was set to "sailfishos".
N9,SYM: Added check for update functionality.

Closes #23 - Update notification.