PriorityQueue implementation for ceylon.collection
Test for PriorityQueue iterator
Optimize heap creation by bubbling Down only on elements with children
Fix parent / children index calculation logic

Previous index calculation formulas used where right for an array whose
first element index is 1.
They have been adapted to an array whose first element index is 0.
Fix misleading documentation about new capacity calculation.

New capacity is based on needed capacity, not on current capacity.
Merge pull request #214 from loicrouchon/PriorityQueue

Priority queue