Markup changes in README.md
Fix for bug where radio cannot be reinitialised

* Calls to get_radio(...) with different arguments will
return a new radio.
* Added doctest for get_radio
* Added __repr__ for Radio
* Test that radio is running for radio.say() and radio.play()
* Added RadioNotRunningError
Restart radio if killed by keyboard interrupt
Use sox to resample audio for pifm in Radio.play()