Moved _merge_tracks() into MidiFile and made it a property.
Moved file type test to MidiFile._messages.
Added bpm2tempo() and tempo2bpm().
Added note about channel numbers to docs.
Removed types.py and moved the code into midifiles_meta.py.

The module collided with the builtin types module causing confusing errors
if you tried to run one of the modules within mido directly (That should
not work anyway but now it will at least give a more helpful error message.)
Documented bpm2tempo() and tempo2bpm().
Made MidiFile.get_messages() public. (May be renamed before release.)
bpm2tempo() now returns an integer.

Returning a float would create problems for the encoder.