Consider negative differences in events channel as events

The current code breaks if there are two adjacent events and the
first is greater than the second.
Add test

Unfortunately, this currently makes the other test fail.
Address comments
Add detect, consecutive, and min_duration parameters to find_events()
Use min_duration in seconds, improve docs
Add consecutive='increasing' to mimic old behavior
Fix long line
Give expected results for detect='offset', consecutive='increasing'
Don't doctest since data is not available, conform to pep8
Merge pull request #420 from simonster/find_events

ENH: Consider negative differences in events channel as events