r776: removed unused variable
added debugging code

Hmm... I am thinking to write a separate error corrector with hash table and
bloom filter. I may have tighter control about what k-mers to add to the hash
table.
added a lock member to khash
allow ambiguous bases with BCR

If there are no ambiguous bases in the input, BCR takes essentially the same
RAM. When there are, BCR may take 50% more RAM at the sequence transpose step.
RAM at the construction step remains the same as before.

Supporting ambiguous bases is important for my future experiement with fermi.
don't cut at ambiguous bases as BCR now works