Improve histogram binning

[jni]

1. See if it's feasible to have the same range (e.g. -4 to +4) for all the histograms, with more bins than currently (I think that the bars are too fat right now)

2. Put the tails in a special wide bin, e.g. anything < -3 goes in a bin from -3 to -4. (while all other bins are 0.2 wide, for example.)

[jni]

Another idea: use optimal binning from astropy:

http://astropy.readthedocs.org/en/latest/api/astropy.stats.histogram.html#astropy.stats.histogram

(currently unreleased, but can install latest from github with pip.)