Refactor hist2d_alex

[tritemio]

From @tritemio on February 9, 2016 17:47

The plot function hist2d_alex is an old complex function with a visualization (2-D histogram + scatterplot) now superseded by the more powerful alex_jointplot.

hist2d_alex should be refactored (and maybe renamed to hist_ES or distribution_ES?) to produce a single E-S "distribution" plot (hexplot, kde, scatterplot) without marginals. Next, this can be used to simplify the implementation of alex_jointplot and to provide a more uniform API for both alex_jointplot and hist_ES.

Copied from original issue: tritemio#43