added halflife to exponentially weighted moving functions
Currently for the exponentially weighted moving functions (ewma, ewmstd, ewmvol, ewmvar, ewmcov) there are two ways (span, center of mass) to specify how fast the exponential decay is. It would be nice to support a "half life" option as well.

The half life is basically just the number of periods in which the exponential weight drops to one half, i.e.,

(1 - \alpha)^h = 0.5, h: half life

can you add an entry in `doc/source/release.rst` (reference this PR, but use the issue tag), enhancements section. thanks
 I've added an entry to the release.rst for this, thanks! 
 gr8. can you squash down to a single commit?
https://github.com/pydata/pandas/wiki/Using-Git
 Sure it's squashed down to a single commit now
 thanks!
