Use sox to resample audio for pifm in Radio.play()
Added Radio.tone() using sox to generate a tone
Added Radio.sox() and refactored tone() play() etc
Added morse code lookup dictionary
Fix for stopping / starting of radio thread

* Should fix bug where get_radio() starts a new radio
    with same settings
* Fixes bug where radio restarts after radio.terminate()
Added Testcases for Radio (tests.py)
Attempted fix on latency issue

* Radio.is_alive() causes nondeterminitic delays of up to several
  seconds when it is called. This patch creates an on_air() method that
  uses Radio.fm_process.returncode instead. However, it appears to have
  the same issue.
Added ' ' to morse code dictionary
Merge branch 'master' of github.com:EWB-Coding-for-Development/ewb
Merge branch 'radio_tone'
Merge branch 'morse'