Issue 24: fixed handling/resolving of parent::method() calls

Changed class hierarchy storage in Analyser from parent->children
mapping to child->parent mapping, which makes parent handling easier
and makes getClassDescendants also more efficient.
Added class hierarchy cycle detection for Analyser::getAncestors to avoid endless loop
Merge pull request #25 from soxofaan/issue-24-parent-double-colon-handling

Issue 24: parent double colon handling