Add clear method to TileLoadQueue.

Also add tests for TileLoadQueue.
Rename TileLoadQueue.js to TileLoadQueueSpec.js.
Add doc for TileLoadQueue.clear.
Merge pull request #443 from AnalyticalGraphicsInc/tileLoadQueue

Add clear method to TileLoadQueue.
Merge remote-tracking branch 'origin/master' into terrain

Conflicts:
	Source/Scene/TileLoadQueue.js