Please let user select the Android SDK Platform target version to build against #4

In https://github.com/groupsky/generator-android/blob/master/app/templates/project.properties#L15 , the target Android SDK Platform version is hardcoded as "android-19", which causes a subsequent (Ant) build to fail if that target isn't available, requiring the user to either:

• install a specific version of the Android SDK Platform
• or else, either:
  • run android update project -t <target-id> --subprojects -p . (huh? whaddayamean?)
  • go in and edit target= in project.properties (assuming s/he knows what to put there)

I think it's better to run android list target and parse its output, and (if there are multiple versions) present the user with a choice among the versions that are actually installed on the system.