Add mRNAseq rules in Makefile.

  * New dependency: BLAT
  * Remove PBS account from header.pbs, each job should be explicit
    about using it.
  * Refactor scripts/blat_merge_outputs.py
  * Add scripts/extract_reads.py for extracting unmapped reads from a
    BAM file, outputting a FASTA file.
  * Remove old PBS submission scripts.