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
you don't have to compile for each 2.10.x version, just use the last 2.10.4 and that binary should be compatible with all other scala 2.10.x versions