API: add ddof to expanding/rolling_cov()
Closes #8279 
closes #8064

CC @jaimefrio
 @seth-p any reason you can think of why you would NOT want to have a ddof? (prob just an oversite when this was initially created)

@jorisvandenbossche 
 I can't think of one. Note that `np.std/var()` has a `ddof` argument, though there it defaults to `0`, whereas I think in Pandas it should default to `1` (for backwards compatibility reasons, if nothing else).

As I observed in #8064, `rolling/expanding_std/var()` do already support a `ddof` argument, but it's just not documented. (And it's not obvious to me how to add it to their doc strings.)
 I also see no reason.

For adding it to the docs of rolling_std et al, I think you have to make a seperate version of `_rolling_func` (https://github.com/seth-p/pandas/blob/rolling_cov_ddof/pandas/stats/moments.py#L628)  for the ones that have a ddof argument.
 @seth-p you may have to split doc templates (or parameterize a bit more)
 is their a reason shouldn't add to rolling_var,rolling_std (and the expanding versions)?
 Those functions already support it (through kwargs, I believe; am not in front of computer), though it isn't documented. See in test_moments.py where they are called with ddof=0.

> On Sep 17, 2014, at 8:34 AM, jreback notifications@github.com wrote:
> 
> is their a reason shouldn't add to rolling_var,rolling_std (and the expanding versions)?
> 
> —
> Reply to this email directly or view it on GitHub.
 needs rebase - give a check, I only think the docs show up for rolling_cov (not the others)
 Correct. While `expanding/rolling_var/std` have supported `ddof`, it hasn't been documented (cf. #8064). Since `expanding/rolling_cov` did not support `ddof``, as part of this PR I added mention of``ddof``to their doc strings when I added the``ddof``argument to them. Still need to add mention of``ddof``in the documentation of``expanding/rolling_var/std``. Unless you prefer otherwise, I will try to do so as part of this PR, which will then also close #8064.
 @seth-p ahh, yes that is good, thxs.
 Rebased and added  `ddof` to the `expanding/rolling_var/std` doc strings.
 looks good, ping when green
 Green.
 @seth-p thanks as usual!
