Add enforcement check for <application> element in <manifest>

 which is declared as "must contain" here:-

http://developer.android.com/guide/topics/manifest/manifest-element.html

This fails fast and prevents the cryptic NPE when parsing application meta data when there is no application.

Addresses https://github.com/robolectric/robolectric/issues/1293

Breaks a lot of tests at Google, we'll just need to add the missing <application> element for all these library projects.
Unused?