Launch app correctly and allow alternative launch activity

Hi,

There seems to be no `-n` option in the [current adb docs](http://developer.android.com/tools/help/adb.html), so after trying a bunch of options, I was able to get my app to launch by changing the argument order to adb shell am start as the commit does.  I also added an extra parameter to allow starting an app with a non-standard entry point activity (as my Cordova-based app requires).

I don't think this will break anyone's build process but it might be worth some testing.

Let me know.