Mozbench android

Enabling mozbench on Android. 
Main features:
* **AndroidRunner** to manage running benchmarks on Android
* **install_fennec** to handle Fennec installation
* **--use-android** argument to specify Android benchmarking and hold device id

I think we want to force people to specify --firefox-url even if they specify --use-android. I just accidentally ran the tests against a year old version of fennec because I forgot to specify --firefox-url.