Add SBT build files with cross-building.

Hi Alex,

I just discovered Skiis when searching for a parallel stream library.
I've added the necessary files to cross-publish for 2.10 and 2.11 using SBT (don't know how to do that with buildr).
It would be great if you could publish the JARs to an official Maven/Ivy repository so that the lib can be used by just adding a binary dependency.

Thanks!
  Martin