Handle "today" and "now" in cython instead of C
ATM the `Timestamp` constructor (specifically `tslibs.conversion.convert_str_to_tsobject`) catches "now" and "today" before passing strings to `_string_to_dts`, which eventually dispatches to `np_datetime_strings`.  By contrast, `to_datetime` (specifically `tslib.array_to_datetime`) does not handle "now" or "today" internally.

This PR makes `array_to_datetime` handle "now" and "today" internally, removing a big chunk of np_datetime_strings in the process.

NB: This does _not_ change the fact that `to_datetime("today") != Timestamp("today")`, just makes this fact more obvious.  My hope is that making this obvious will make it easier to change/fix in the future.

asv results are as usual all over the place, but if anything look slightly faster.