IntDisjointSets: use bounds-checking on user-supplied indices
README: DisjointSet -> DisjointSets
Deprecate add\! in favor of push\!
Update README
Merge pull request #47 from lendle/pull-request/5c05539e

Deprecate add! in favor push!
Merge pull request #46 from timholy/pull-request/8d8ae5f9

IntDisjointSets: make indexing safer