Added make_set to add single element as a new disjoint set, with its parent equal to itself
Added ! to the name of the make_set function, since it modifies the structure
Added version of make_set! which automatically chooses the new element as the next available one
Changed length(s.parents) to length(s)
Added make_set! for arbitrary typed DisjointSets
Added make_set! to exports in DataStructures.jl
Added working test of make_set!
Changed the name make_set to add_singleton
Merge pull request #19 from dpsanders/MakeSet

Add make_set! to add a new set with a single element for IntDisjointSets and DisjointSets