Add stats benchmarks

[bsipocz]

E.g. sigma clip was being reported very slow here:

astropy/ccdproc#624

[larrybradley]

I plan to add some sigma clip benchmarks soon.

[larrybradley]

Sigma clipping benchmarks: #55

[pllim]

@larrybradley is this solved or there are more?

[bsipocz]

@pllim - Currently we have the sigma clipping benchmarks, but ideally it would be great to add benchmarks for other stats functionality, too.