Pypianoroll v0.5.0

• Add pypianoroll.metric module
• Support more options in pypianoroll.parse
• Rename methods for Multitrack
  • get_maximal_length -> get_max_length
  • get_num_downbeat -> count_downbeat
  • get_stacked_pianorolls -> get_stacked_pianoroll
• Improve the documentation (mostly docstrings)