PERF: Speed up pd.to_datetime() by optionally inferring dt format

Given an array of strings that represent datetimes, infer_format=True
will attempt to guess the format of the datetimes, and if it can infer
the format, it will use a faster function to convert/import the
datetimes.  In cases where this speed-up can be used, the function
should be about 10x faster.
PERF: Add infer_datetime_format to read_csv()

This allows read_csv() to attempt to infer the datetime format for any
columns where parse_dates is enabled.  In cases where the datetime
format can be inferred, this should speed up processing datetimes
by 10x.