Add Android input method methods (Appium 1.2)

[imurchie]

Add methods to query and set the input method (IME) in Android (spec). Five methods are there:

1. available engines: returns a list of engines available on the device
   • GET /wd/hub/session/:sessionId?/ime/available_engines
   • wd
2. active engine: returns the currently active engine
   • GET /wd/hub/session/:sessionId?/ime/active_engine
   • wd
3. is active: returns boolean, IME is active or not (on Android this is always true)
   • GET /wd/hub/session/:sessionId?/ime/activated
   • wd
4. activate engine: takes the package and activity, and activates it
   • POST /wd/hub/session/:sessionId?/ime/activate
   • payload: { 'engine': 'com.android.inputmethod.latin/.LatinIME` }
   • wd
5. deactivate engine: deactivates the currently active IME on the device
   • POST /wd/hub/session/:sessionId?/ime/deactivate
   • wd