Add a DataStructures module with PriorityQueue and heap functions.
Rename DataStructures to Collections. Document.
Export Collections, not DataStructures.
"DataStructures" to "Collections" in a comment.
More specific type for index in PriorityQueue.
Rename has to haskey in PriorityQueue.
Merge pull request #2920 from JuliaLang/dcj/pq2

Add a DataStructures module with PriorityQueue and heap functions.
pkg2 wip: single-package methods expect only availability for it.
fix Gunnar's issue 3 and part of 4