Do not require the presence of a manifest for library projects.

Be sure to warn that this is the behavior we are falling back to.
Allow manifest to be specified from @Config annotation.
Merge branch 'JakeWharton-jw/library-projects'

* JakeWharton-jw/library-projects:
  Allow manifest to be specified from @Config annotation.
  Do not require the presence of a manifest for library projects.