Add configuration setting to control sending of messages if not /away.

The script currently only sends messages if the user has set themselves /away. 

Personally, I'm rather lazy, and never bother with /away .... so I've introduced a new setting (pushover_only_if_away) which controls whether we have to be away or not before sending notifications, and a new function (check_away()).

Request includes minor refactoring of code (hopefully making it easier to read). 

Other commit includes a re-indentation of the ignore_channel bit ... this seemed to be at 8 spaces or something. I'm too stupid to figure out how to split this into a separate pull request (Sorry).