ENH: Allow exponential weighing functions to specify alpha, in addition to span / com / halflife
Currently the exponential functions take one of three arguments to specify the length of backhistory. Each of these numerically converts to alpha. Outlined here: http://pandas.pydata.org/pandas-docs/stable/computation.html#exponentially-weighted-moment-functions

Is there a reason we don't allow users to just pass in `alpha`? This is how I think about weighing some of the time.

I'm happy to do a PR in time; this issue is to solicit feedback.
