More robust tempo handling in midi files.

Added a check to ignore multiple identical tempo changes in a midi
file.
Theoretically, more than one tempo change can not occur at the exact
same time. However, if there are multiple tempo changes at the exact
same time _but with the same target tempo_ there is not really any
reason to bail out.
Merge pull request #101 from gebart/midi-parser-bugfix

More robust tempo handling in midi files.