Fix bug in guitar strum direction handling.  Both :up and :down sounded the same!  How did I not notice this before?

Also update code to be more tolerant of odd-sized vectors when specifying frets via the-chord.  For example, a vector of size 3 will just play the top 3 strings, a vector too large will just be truncated.
Merge pull request #240 from rogerallen/rallen-guitar-3

Fix bug in guitar strum direction handling.