Add convert4crosstab, for converting genotypes to form useful for crosstabs

 - This is a bit too much like pxg2json; but here I don't want imputed
   values, so I just started from scratch.
convert4crosstab: include check of genotype codes
panelutil: crosstab, rowSums, colSums, matrix transpose
Initial steps toward crosstab panel; not yet giving any display
panelutil: calc_crosstab: include marginals and total in the result
panelutil.css: style for text in crosstab cells
crosstab.coffee: a few more steps towards the display
crosstab: use x and y scales; gray background for key bit
crosstab: I had shaded too much
in crosstab, use "N/A" for missing rather than "-"
crosstab: add some borders + column headings
A bit of gratuitous interaction, for now
crosstab: Add some real interactivity, to give conditional distributions
crosstab/ReadMe: clarification regarding missing value category
crosstab: color options, in ReadMe and in coffeescript code
remove xlab and ylab options and make them part of the input data
crosstab: Messing about with labels
crosstab/test/index.html: Add caption
crosstab: Add marker names as row and column headings
crosstab: forgot to update crosstab.js