provide exceptions from database to client
adjustments & fixes in exception handling
publish() should also accept beforeRequest validator
Reworked validators to return Futures
published collections should be private to Publisher & fix tests
removed FutureMock
Merge remote-tracking branch 'origin/master' into exception_handling

Conflicts:
	test/test_all.dart
Merge pull request #23 from cleandart/exception_handling

provide exceptions from database to client