Add regexp support to the message matcher

- Move the Matcher out of the pipeline package to message
- Rename FilterSpecification to MatcherSpecification
Fix the aggregate output in CounterFilter

The interval can exceed the test condition when there is no input
causing the aggregate to never be flushed again
Merge branch 'dev' into features/message_matcher

Conflicts:
	pipeline/all_specs_test.go
Merge pull request #43 from mozilla-services/features/message_matcher

Features/message matcher