Add a method allowing one call to either add an operation or adjust the priority if it already exists in the queue.
Don't change an operation that is already executing.
Rather than modify an already queued operation, cancel the existing operation if the priority has changed and it's not already started, and add the new operation with the new priority.
Change brace style to match existing code.
Merge pull request #1 from mbaltaks/master

Add new method for adjusting priority of queued operations.