Initial patch to Multiplexer to add a second path through.
Max age work.
Adding support for delegating request handler.
BranchingRequestHandlerBuilder.
Tests for BranchingRequestHandler.
Fix some stuff.
Merge branch 'master' into no-rpc-age

Conflicts:
	test/all_tests.dart
Comments for code review.
Merge branch 'master' into no-rpc-age
addBranchForAll() fix to use forEach().
Merge pull request #52 from alxhub/no-rpc-age

Add a split codepath in Multiplexer, which support a noRpcAge parameter. Add a BranchingRequestHandler which can route a request to a particular backend, with a fallthrough.