Update Scala, sbt, xsbt-web-plugin

Update to
* Scala 2.10.4 (crossScalaVersions "2.10.1", "2.10.2", "2.10.3", "2.10.4", maybe this can be reduced)
* sbt 0.13.5
* xsbt-web-plugin 1.0.0-M6
is this not needed in 0.13.x or why are you removing it?