Support for sbt 0.12

By:
 * using the new [sbt-cross-building for sbt 0.13](https://github.com/jrudolph/sbt-cross-building/issues/23) (currently only as SNAPSHOT version available)
 * the [new futures-backport](https://github.com/jrudolph/futures-backport) library containing SIP-14 for Scala 2.9.2
 * the [new bintry for Scala 2.9.2](https://github.com/softprops/bintry/pull/6) (which has yet to be published for this to work)
 * a few changes to make the code compatible with sbt 0.12 / scala 2.9.2

This fixes #8.

In practice, there's a gotcha that bintray-sbt won't work together (`java.lang.IncompatibleClassChangeError: Implementing class`) with plugins depending on older dispatch versions which AFAIK for 2.9.2 means all plugins using dispatch because there's no 2.9.2 version of dispatch 0.11.0 because the futures-backport was previously missing from the picture.

Proof of concept: https://bintray.com/jrudolph/maven/sbt-dependency-graph