PrettyMIDI.adjust_times should force times to be monotonic

[craffel]

Right now, there is no check/enforcement that original_times and new_times be monotonically increasing. This results in very strange behavior, like negative tempi, which causes later issues. They should be tested for monotonicity; if they are not monotonic, a warning should be given and they should be made monotonic with np.maximum.accumulate.

[areeves87]

I've taken a crack at this issue with the above pull request. #145

I inserted a private method within PrettyMIDI.adjust_times called _strictly_increasing() that enforces monotonicity and throws a warning if it detects a violation.