Change how the "no run-main for play" hack works

Now we convert "run-main play.core.server.NettyServer"
into "run", rather than ignoring main classes for apps
with the Play dependency.

This new approach has two advantages:
 - if there is another main you can pick it
 - doesn't rely on getting hasPlay right, in case
   that check is broken
Merge pull request #210 from typesafehub/wip/havocp-change-play-run-hack

Change how the "no run-main for play" hack works