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

If you run mozbench.py -h, this shows up as:
  --use-android [USE_ANDROID].
It would be nice if this was:
  --use-android [DEVICE ID]