add bisect function

distanceFromIntersectArea wasn't working in some cases (see
https://github.com/benfred/venn.js/issues/3).

The problem was caused by using an unconstrained function minimizer to compute
this distance - which went awry when the solution was near one of the
constraints (i.e completely overlapped or completely disjoint).

Changed to compute this using bisection
fix
2 more examples from original blog post
Merge pull request #5 from benfred/add_bisect

Add bisect function