Expose functionality to check if app is installed in new function is_installed

Fixes issue #7.
you need to check if adb_path is None and use 'adb' if not due to https://github.com/mozilla-b2g/fxos-appgen/blob/master/fxos_appgen/generator.py#L75 or change every adb_path=None assignment to 'adb'. I prefer the latter.