Issue 18: support for parent methods

This pull request fixes issue #18:
functions defined in parent classes can now be detected as used

The pull request consist of two commits:

A first one to split up code parsing and dead code detection logic (in separate classes: Analyser and Detector).
There is no functional change here, it only makes it easier and cleaner to fix issue18 (and allow future improvements) 

A second commit to actually fix issue 18

I hope that splitting this up in two commits makes review also easier