Upgrade to sbt 0.13.1.

Upgrade the following plugins:

  * sbt-scalariform
  * sbt-osgi
  * sbt-assembly
  * sbt-revolver

Remove xsbt-web-plugin because I couldn't find the right version. Remove
all references to xsbt-web-plugin from build file.

Add missing argument to StreamCopier constructor:

[error] /Users/grek/scala/spray/project/Utils.scala:46: not enough arguments for constructor StreamCopier: (bufferSize: Int)com.decodified.scalassh.StreamCopier
[error]     new StreamCopier().emptyToString(getClass.getClassLoader.getResourceAsStream(resource))