ENH : bin_edeges helper function

Adds helper function + tests for generating bin edges from
any sub-set of the maximum, minimum, step, and number of bins.
DOC : clarified documentation on bin_edges

 - explicitly specify left edges
 - fix typo
 - spell out constrain math in English rather than symbols that only
   make sense to me
TST : made variables in test-code more meaningful
BUG : fixed typo in exception
DOC : warning about (range_min, range_max, step)

 - used warning call-out in docstring
 - added debug-level logging when the right bin edge
   is not equal to range-max
Merge pull request #55 from tacaswell/bin_limits

ENH : bin_edeges helper function