add bloom_filter module

See the comment in bloom_filter.h for info on what a Bloom filter is. I plan to
use this in the IVS tcam module, as well as to replace an existing open-coded
Bloom filter in the IVS upcall processing code.
Merge into master from pull request #23:
add bloom_filter module (https://github.com/floodlight/bigcode/pull/23)