RF/ENH: find_events

RF: first find all steps, and then extract events from steps
ENH: also return second column in output
FIX Epochs: Don't exclude events where the second column != 0
ENH find_events: work on Raw instead of array
ENH find_events: add `detect='step'` option
ENH find_steps: add merge parameter
ENH: expose find_steps()
TEST: find_steps() merge parameter
FIX merge_events(): take into account second column
ENH find_events: use merge find_steps parameter for min_duration
FIX: find_steps() pad_start parameter
DOC: clean up sentence fragment
FIX: find_events/find_stim_steps

 - removed outdated `first_samp` kwarg / updated docstring
 - don't break if no events are found
   - added more tests
 - renamed find_steps -> find_stim_steps
 - updated whats_new
rename `find_events()` kwarg `detect` --> `output`