Locators for testable objects

Implement locators for functions/classes/methods to allow more fine-grained test generation. Currently pythoscope takes list of modules as arguments. This change would allow the user to pass locators like "module:SomeClass.method" or "package.module:some_function" to generate only test cases for given objects. This change could also solve the "should we test inner classes" problem. We could go by *not* generating them by default and allowing to invoke the generation by passing an explicit locator for the inner class. The same goes for other ignored objects, like private methods or exception classes.