Added generate_kmers.py

This little script will output all the possible kmer permutations to
stdout. The only input is the kmer length desired.

Here are some examples.

output 2mers
  ./generate_kmers.py 2 > 2mers.py

output N-mers up to 6 with a bit of bash-foo
  for i in `seq 6`; do ./generate_kmers.py $i > ${i}mers.txt; done;
simplify generate_kmers.py