support for sbt 0.13; introduced sbt-cross-building

Hi,

To get sbt 0.13 support in sbt-cxf I introduced the cross building plugin ( https://github.com/jrudolph/sbt-cross-building ).
It can be used to publish sbt-cxf for sbt 0.11, 0.12 and 0.13 by running '^ publish'.

I hope this makes sense to you.

Cheers,
Enno.