Use version 0.13. Update build.sbt to cross compile.

Change xsbt-gpg plugin to sbt-pgp plugin as the former is not
available with the current version of sbt.

Update build.sbt to update Scala 2.9 patch version and to
cross compile with versions 2.9.2 and 2.10.3.

I did not found any incompatibility so the project should cross compile 
properly.
The cross build and artifact publish should work with 

```
sbt +compile
sbt +publish
```