prebuilding the selendroid apk once per package and caching it locally.

assumes that 'locally' is the box which is signing with it's debug certificate
Checking for system environment variables and system path for dependencies.

Fixes Issue 2.
need to import os
be able to run this from anywhere, turn off the debugging
throw an exception (earlier) if it fails to install
updating readme with info about app.package/main_activity