Add Choose.xmap

Choose.xmap provides a simple way to create a Choose instances from an
isomorphic type with a Choose instance. For example, to create a
Choose[Date] based on the existing Choose[Long], one could write:

```scala
implicit val chooseDate = Choose.xmap[Long, Date](new Date(_), _.getTime)
```
When testing, set the Future thread pool size to Test.Parameters.workers, instead of using default number of threads.
Inspired by the recent commit from @ceedubs [1] which adds xmap to Choose, this
commit adds a similar xmap to Shrink which provides a way to create a Shink
instance from an existing Shrink instance of an isomorphic type

[1] https://github.com/rickynils/scalacheck/commit/c87978b52a8ce11a519fdf30ad543eda60d47507
Don't do exit(0)
Bump Scala version from 2.11.0 to 2.11.1
Bumb sbt version from 0.13.2 to 0.13.5
Test: Properly shutdown thread pool
build.sbt cleanup + sbt-pgp upgrade
Fix for #104 Support verbosity under sbt for TestFrameworks.ScalaCheck
Bump version to 1.11.5-SNAPSHOT
Remove test for Prop.within. Fixes #106 (and Travis builds, hopefully)
Fix issue #89
Revert "Inspired by the recent commit from @ceedubs [1] which adds xmap to Choose, this"

This reverts commit 9ede3b2d418dfb134f85727543357fc7d8465243.

Not compatible with Scala 2.9.3.
ScalaCheck 1.11.5 release.
Revert "Revert "Inspired by the recent commit from @ceedubs [1] which adds xmap to Choose, this""

This reverts commit 54e9d8179f5405720f73b83c68ae3177b6990c61.
Manually cherry-pick relevant changes from master

(cherry-picked from 0f0367ba0d7af6c70196b159b89c9a76c95c1079)
ScalaCheck 1.11.6 release
Merge branch '1.11.x' into master

The 1.11.x branch was a temporary thing that should be removed when we are back
on a normal release-from-master procedure. This merge makes sure we don't lose
the commits that were made for 1.11.5 and 1.11.6.
Update release notes for ScalaCheck 1.12.0
Scala 2.11.4