fix(injector): untyped constructor parameter should throw proper error

Fixes #41
Merge branch 'noconstructor' of git://github.com/vicb/di.dart
Make keys immutable
chore(travis): initial travis setup
Added a few more tests to make sure keys are immutable
Merge remote-tracking branch 'origin/master' into feature/support-for-annotations

Conflicts:
	test/main.dart