added text to readme + small cleanups

I added a description to readme + small cleanups (like using implicit classes instead of implicit defs when possible)
This is OK (I think) - see the [scalacheck example](https://github.com/rickynils/scalacheck/blob/master/examples/simple-sbt/build.sbt) 

The reason that scalajs is not set up like this is just a [scalajs limitation]( https://github.com/rickynils/scalacheck/tree/master/examples/scalajs#limitations)