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
From the change it looks like we are changing the behavior. It seems before we were able to write something like:
appbuilder emulate android arg1 ImageArgument (the second argument to be used for image)
but now it is not possible anymore. If the previous behavior was not intended, I'm ok with the change.