PERF: improve to_timedelta perf for string-like
see #6754 
related #3009, #8190 
from [SO](http://stackoverflow.com/questions/30894267/defining-a-function-pandas)

convering string like timedeltas, e.g.

'1 days', '3 days, 00:00:03', '3 days 2 hours'

is slow because it requires a regex match for each element. Can put cythonized (like format parsing for datetimes), or since only very limited formats supported can simply test them (in cython) and convert.
