ENH: More informative error messages
FIX: Minor adjustment
FIX: Only need to check if str
FIX: Minor change
FIX: Making changes simpler
PEP8
Merge pull request #357 from Eric89GXL/evoked-errmsg

ENH: Improving Evoked error message
ENH: define higher order events

- define_events function for creating new events via time lag between exisiting events
- example: plot fast motor responses
COSMITS + ENH: adding test, fixing doc string
ENH: fixing integer issue + improving example
COSMITS
COSMITS: updating doc text.
ENH + COSMITS: addressing points from review, improving example

- adding subplot
- changing name to define_target_events
- adding test doc string
- returning empty array instead of None
- moving function to event name space
COSMITS: units