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.
I'm not sure that you need all these tests if all they do is raise errors.
Doing something in the lines of : 
```
try:
    filename = '{}/genes.fpkm_tracking'.format(os.path.realpath(dirname))
    samples[parse_filename(os.path.basename(dirname))['sample']][match['proportion']] = filename
except IndexError:
```
would have the same result in case of errors (raise error, if you want to just change the message you can except/raise again) but will not check every time. I think that might be part of python best practices or something, but don't quote me on that.