Possible fix for the lowest_common_hypernyms

Currently lowest common hypernyms do not return a result based on the
hypernym paths of the 2 synsets. So there may be a "better" result when
taking into account the hypernym paths.
Consolidate changes into single function

Based on suggestions from Tschier to use max_depth() function
Updated the docstring
Merge pull request #415 from dougalg/master

Proposed resolution for issue 395