more documentation; add test command; move app api key to token; make pushover sound configurable

Hi -

I've updated the documentation (README.md) for the project to help people install it.
I've also made the script a bit more configurable - e.g 
 * making the pushover app and user tokens configurable, 
 * allowing the user to specify the sound to use, and
 * added a /pushtest method to test (helpful when installing, I hope).

Thanks
David.