[WIP] Fix unit conversions

[superbock]

This PR addresses mido#102 and adds the necessary time signature arguments to the functions in mido.midifiles.units and modifies MidiFile.__iter__ to also respect time signature events in a MIDI stream/file.

This PR furthermore adds a couple of other (related) things:

• add option to MidiFile to choose the timing unit to be used (ticks, seconds, beats),
• add option to MidiFile to choose between relative and absolute timing when iterating over messages.

Before being able to be merged, these points should be discussed/addressed:

• Extend the documentation to reflect all introduced changes, especially update the image in midi_files.rst.
• Import the new tick2beat and beat2tick into the main namespace?
• Include tick2beat and beat2tick in the API docs.
• MidiFile.length raises a ValueError whereas MidiFile.__iter__ a TypeError in case of type 2 MIDI files. Which one is more appropriate?
• Add docstrings, but which docstring format to use?
• renaming ticks_per_beat to ticks_per_quarter_note or add new variable (how to deprecate the old one)?