Add a subclass of CallablesFilter that also implements the DependencyExtractor interface.

Useful for tests, among other things.
Fix totally screwed code >:-)

Better run once before push.
Fix return value if no extractor given
Merge DependencyExtractor callable into baseclass
Add a test
Coding standards
Merge pull request #431 from mpdude/callables-dependency-extractor

Add CallablesDependencyExtractor