sbt 0.13 variant of scrooge-sbt-plugin

Before this change (scrooge 3.12.3) only sbt 0.12 variants (targeting scala 2.9.2 and scala 2.10) of scrooge-sbt-plugin were being published.

This patch adds scala 2.10 sbt 0.13 variant of scrooge-sbt-plugin. It is made possible through use of sbt-cross-building plugin, detaching scrooge-sbt-plugin from aggregated scrooge build, and bumping sbt launcher version from 0.12.1 to 0.13.1.

As consequence:
+ scala 2.10 sbt 0.12 variant of scrooge-sbt-plugin is not longer supported
+ CI/release process has to be adjusted to first publish scrooge non-sbt-plugin modules (via regular cross-scala build), and then to publish scrooge-sbt-plugin module (via cross sbt build)