Create UnionFind.podspec

A tiny library implementing a union find / disjoint set data structure, for use as a building block in larger algorithms like incremental connected components, unification, etc.

Manual test with `pod 'UnionFind', :podspec => 'https://raw.github.com/Strilanc/UnionFind-ObjC/master/UnionFind.podspec'` worked without warnings and functioned correctly in a test project.