Create pairs.weighted.r

A lot needs to be done for this one. 

- First, we need to lift the workhorse algorithm for hexbinning (can be found in the hexbin package).

- Second, we need to decide a sensible way to create a "common" cell count between weighted / aggregate groups. One variable may have a great number of duplicated response levels, so we would like to know that if the really dark cell represents, say "1000" counts, the really dark cell in the next plot doesn't represent only "120" counts.

- Third, we need to arrange them so they can be plotted in a panel as mentioned earlier.

Some thoughts:

Weighted hexplots or "wexplots" seems like a good idea in its own right. I believe I'll make that it's own function.