Prevent multiple tick conversions
Prevent multiple tick conversions

Adds internal state and checks to Pattern and Track to prevent duplicate
calls to make_ticks_abs() and make_ticks_rel(). Fixes #20.

rlkelly/python-midi,rlkelly/python-midi,ListFranz/python-midi,shimpe/python-midi,ListFranz/python-midi,shimpe/python-midi,eli-b/python-midi,vishnubob/python-midi,pdorrell/python-midi,shimpe/python-midi,chaosmail/python-midi,jameswenzel/mydy,chaosmail/python-midi,pdorrell/python-midi,vishnubob/python-midi,ListFranz/python-midi,jameswenzel/mydy,eli-b/python-midi,vishnubob/python-midi,jameswenzel/mydy,eli-b/python-midi,rlkelly/python-midi,pdorrell/python-midi,chaosmail/python-midi