A little cleanup; no code changes
OPEN gh-21: Ongoing performance monitoring 
http://github.com/TheFeshy/mp3utensil/issues/issue/21

Improved typical speed of python implementation by adding a second
method for identifying potential frames.  However the new method is
drastically slower in atypical cases (scanning non-mp3 files) so both
methods are used.  Test cases to verify both methods are working were
added (along with a few other tests)