Fixes to work against scala 2.11's modularization

Scala 2.11 splits out parser combinators as its own library.  To depend on this library, you need to explciitly add the dependency, similar to what happened when xml split off.  Here's an updated build which can take 2.11 jars and still works against previous scala versions.  You can start dropping special cases when you drop support for old scala versions.

Thanks again for a great library!