Add feature from issue #256.

I wasn't really sure where to put this stuff. Everything seems to be stuffed to willie.bot and this isn't very general so I put it there. It could also be in its own module though.

I tried to be careful with the threading, but that is always a bit tricky so feedback would be appreciated.

Relevant changes:
bot.py:
-Add new thread for calling functions that have interval defined.
-Add handling for the new interval-attribute.

module.py:
-Add decorator @interval.