arbirtrarily spaced bin grids

This changeset adds the ability to specify an arbitrarily spaced bin grids, using a binary search into the grid to get the index.

It is functionaly complete except for documentation, which I'll work on subsequent to any changes you'd like to make (I hate having to write documentation twice).  I plan on releasing a companion module Math::Histo::Grids to aid in generating more commonly used grids.