Add cross sbt build support

This patch adds support for building cross sbt variants of the sbt-scalabuff plugin, through use of [sbt-cross-building](https://github.com/jrudolph/sbt-cross-building) plugin.

With this patch applied one can e.g. from sbt console issue _**^ publish-local**_ command and have two sets of sbt-scalabuff plugin artifacts published in local ivy repository, one for scala 2.9.2 and sbt 0.12, and second one for scala 2.10 and sbt 0.13.