Add `div` and `and` methods

I need to point out that there's some inconsistency between jsbn and bigint when dividing negative numbers -- jsbn rounds towards zero for integer divisions, e.g. 10 / -3 = -3, whereas bigint rounds towards negative infinity, e.g. 10 / -3 = -4. Positive numbers work fine, therefore the PR. I'm willing to look further into this if required.