Made SlickDDLPlugin much simpler and more robust.

This fixes #57, in particular also the silent failure of the previous implementation, where everything would appear normal if the the SlickDLLPlugin could not find the table objects anymore after a refactoring, but the DDL would not be updated anymore. Now this cannot happen anymore, because if it is enabled it either succeeds or fails with noise. Also SlickDDLPlugin became much shorter in code size.

Warning: This is a source incompatible change.

Improvements:
* more flexible and robust because it communicates with the user app via the play.api.db.slick.AutoDDL singleton object instead of automagical table object discovery
* safer: scalac checks imports and objects referenced in play.api.db.slick.AutoDDL (unlike the existence of a package mentioned in config like it was before)
* less reflection: now only happens in def fetchAutoDDLobject and just a bit
* does not override user written evolution scripts, but shows error message instead