Support union and intersection

Refs: https://github.com/vedang/bloomclj/issues/2

Note that I didn't need to use formulas linked - those are only for the cardinality. 

Since union and intersection have such trivial formulas (if the parameters of the filters are the same - just assumed), I've implemented them directly.

Even if this library wanted to add a `cardinality` function, you'd only need to implement those formulas linked to if you wanted to skip creation of the related intersection / union filter. 

(or possibly the intersection cardinality function is more accurate than the cardinality of the intersected filter?)