stats.effhist - estimate efficiency and uncertainties

This implements a function `stats.effhist()` that is very useful for estimating detection efficiency as a function of some parameter (e.g., magnitude) in Monte Carlo experiments. Importantly, reasonable uncertainties on the detection efficiency are calculated. This is done with a helper function `stats.binom_conf_interval()`.

These functions both require `scipy`.

Built docstrings for the two functions can be viewed here:
http://kbarbary.github.com/astropy/stats/index.html