Contamination rates for fastq_screen corrected

Now FACS and fastq_screen report similar contamination rates, it was a silly error in `test_fastqscreen.py`:

http://nbviewer.ipython.org/github/brainstorm/facs/blob/a669816c/facs/utils/benchmarks_facs.ipynb?create=1

Also, the default threads in FACS seem to be always 1, regardless of `export OMP_NUM_THREADS` environment variable. @tzcoolman, can you check and test this out?

I'll be generating plots for the recall and precision soon.

Solves issue #99.