Make in-memory bloomfilters pickle-able

[stevesimmons]

The bloomfilters don't have reduce and setstate functions and so cannot be pickled. In particular, this prevents in-memory bloomfilters being used in multiprocessing.Pool or in cloud lambda functions, etc.

Do you have any plans to add these?

I guess the easiest way to do it is adding cython functions to read/write _bf.array.

[stevesimmons]

I had a go at solving this here:
#44