Add ability to select a particular avd for $ appbuilder emulate android

Add ability to select a particular avd for $ appbuilder emulate android
Implements http://teampulse.telerik.com/view#item/276157
[...] emulator. If you do not select an Android virtual device (AVD) with the avd option, your app runs in the default emulator or the currently running emulator, if any. To list the available AVDs, run $ android list avd