Add expanding moment functions and related tests.

[qwhelan]

Implement expanding_ analogues to the rolling_ moment functions available in pandas/stats/moments.py by calling the rolling_ function with window=len(arg) and min_periods defaulting to the minimum number of data points to compute the statistic (1 for most, 4 for kurtosis, etc.). Resolves Issue #849.

The only potentially surprising behavior is that some expanding_ functions differ from their NumPy equivalents in whether they preserve NaNs. For example, expanding_max(s) is equivalent to rolling_max(s, len(s), min_periods=1), but both produce different output from s.cummax(), as the latter always returns NaN for every NaN in s. I don't have a strong opinion on which output is correct, aside from thinking rolling_ and expanding_ should agree.

[wesm]

This is great, thanks. I'd held off on adding the expanding window functions but they're useful to have and will reduce the number of questions I get ;) It would be nice to add something to the documentation about them