ENH: autocov function for Series
Add an autocov function to Series.

Default is single lag w/ bias, but those are configurable.

May want to add a see also in the docs for the statsmodels version, sm.tsa.acovf?

Aside: I imagine we're going to continue to see duplicate efforts on all of these little convenience methods.
 I think that's a good idea.  I've updated the docstring.
 I think we are deferring to statsmodels on this...if you disagree pls reopen/open thanks
