New plot tool: subsampled gene counts

Added a new tool - take a group of BAM files (typically RNA-Seq), subsample them, run cufflinks on all of the subsampled files and plot the number of genes observed versus the subsampled read counts.

Designed to give a more intuitive / biological angle on the preseq complexity plot.
If you like polite code, try this: https://www.addedbytes.com/blog/if-php-were-british/