added a method to allow writing `file must not exist`. fixes #297

Conflicts:
	matcher-extra/src/main/scala/org/specs2/matcher/FileMatchers.scala
	tests/src/test/scala/org/specs2/matcher/FileMatchersSpec.scala
collect NotImplementedErrors in AllExpectations. fixes #298

Conflicts:
	common/src/main/scala/org/specs2/execute/ResultExecution.scala
	core/src/main/scala/org/specs2/specification/ResultsContext.scala
	core/src/main/scala/org/specs2/specification/process/Stats.scala
	core/src/test/scala/org/specs2/specification/AllExpectationsSpec.scala
	junit/src/main/scala/org/specs2/matcher/JUnitExpectations.scala
	junit/src/main/scala/org/specs2/reporter/JUnitReporter.scala
	matcher/src/main/scala/org/specs2/matcher/MatchResult.scala
	matcher/src/main/scala/org/specs2/matcher/Matcher.scala
	matcher/src/main/scala/org/specs2/matcher/StandardMatchResults.scala
re-throw exceptions when using the Result.unit method. fixes #299

Conflicts:
	core/src/test/scala/org/specs2/mutable/MutableSpecificationSpec.scala
added paragraph descriptions in s2 strings