added a function to export the bloom and to size based on number of items and error rate

    based on http://hur.st/bloomfilter
    numItems = Number of items in the filter
    p = Probability of false positives, float between 0 and 1 or a number indicating 1-in-p
    m = Number of bits in the filter
    k = Number of hash functions

default err rate is 1 in 1 mil