Backwards logic in newer check.

Regen .ant_targets only if it's newer than build.xml? Yeah, that seems wrong.
Change ant target enumeration

Make use of ant's project help feature instead of trying to parse XML with regex (see: http://stackoverflow.com/a/1732454/740787).
This is a behavioral change that does a few things:
 • adds support for ant imports, which were previously not recognized as possibly containing targets
 • supresses targets with no description, as these are conventionally for internal use only