Remove Gemfile.lock from project repo
Add thread-safety test for PriorityQueue#push
Fix thread-safety for PriorityQueue#push
Add thread-safety test for PriorityQueue#shift
Fix thread-safety for PriorityQueue#shift
Fix thread-safety for PriorityQueue#peek/delete
Reorganize PriorityQueue a bit

Meaningful changes are:
- heapifying an appropriate sub-heap when deleting
- removing a bunch of redundant conditions
Use nil to indicate end of PriorityQueue
Merge pull request #15 from brainopia/master

Finish up PriorityQueue