Added exist matcher support, using Existence type calss.
Added readable matcher support, using Readability type class.
Added writable matcher support, using Writability type class.
Initial work to make enabler type classes contravariant.
Deleted commented-out code that is no longer needed because of the contravariance.
Fixed names of implicit producers in Sortable companion object.
Made Sequencing contravariant.
Maded Sortable contravariant. Wrote a test for Collecting, but it
already passed so didn't change variance of Collecting.
Merge branch 'existence' of https://github.com/cheeseng/scalatest
Merging in Chee Seng's readability and writability enhancements with conflicts fixed.

Conflicts:
	project/GenFactories.scala
	src/main/resources/org/scalatest/ScalaTestBundle.properties
	src/main/scala/org/scalatest/matchers/Matcher.scala
	src/main/scala/org/scalatest/words/MatcherWords.scala