Add filter and filterAll methods to filter keys based on a predicate
New tests for subset, subsetAll, filter and filterAll
Use an intermediate JsValue. Make more things publicly visible.
Merge branch 'master' of github.com:pauldijou/play-jsmessages into pdi-master

Conflicts:
	jsmessages/src/main/scala/jsmessages/api/JsMessages.scala
#25 perform filtering and data formatting only once if possible. Implement subset in terms of filter.