Upgrade to Scala 2.11.0-M7 and Scalacheck 1.11.1

A few TODOs remain:
  - implicit search seems to have changed slightly (see 1768ea1) /cc @retronym
  - Scalacheck 1.11 has a new approach to specifying how many test may be discarded 

I'm also preparing an update for your SBT build, so that Scalatest can be built as part of our community build.

I haven't submitted this to the main repo due to the unresolved TODOs. It should probably also go to a separate branch, say scala-2.11, or else conditional logic needs to be added to build.xml.