Add DataStructures package

This is created based on the discussion here:
https://groups.google.com/forum/?fromgroups=#!topic/julia-dev/qjwZBvD70T8

Package readme is here:
https://github.com/lindahua/DataStructures.jl.git

It now implements Deque (based on block list), Stack, Queue, Disjoint Sets, Binary Heap, and Mutable Binary Heap. I took care of the performance in the implementation to make sure that they perform reasonably fast. Graphs.jl (and perhaps others) may rely on these data structures for some algorithms.

I am aware of the debate on where data structures should go. But, until we come to a conclusion, these things may just stay here. We can move them elsewhere when we have a clear plan.