Topic/eclipse setup

Just the addition of the (typesafe's) [sbteclipse](https://github.com/typesafehub/sbteclipse/) plugin which allows generation of Eclipse project metadata with a single command.
Afterward, importing into Eclipse can be done as usual.

I added few lines of instructions to the README.

This is the way I have been using Leon. Let me know if this does not work for you or you have some suggestions.