Refactored function declaration/usage analysis to own class.

This commit is preparation to make future
improvements (and unit testing) easier.
It just moves the source code parsing and function
declaration/usage analysis to a dedicated Analyser
class and leaves the unused function detection logic
in the Detector class.
Issue #18: add detecting function calls to a parent methods
Merge pull request #23 from soxofaan/issue-18-parent-methods

Issue 18: support for parent methods