Move gitignore for Thumbs.db to the root to get rid of two other gitignore files.
Do not unload tiles with transitioning imagery.
Add clear method to TileLoadQueue.

Also add tests for TileLoadQueue.
Rename removeTile to shouldRemoveTile.
Merge pull request #442 from AnalyticalGraphicsInc/tileReplacementQueue

Do not unload tiles with transitioning imagery.
Rename TileLoadQueue.js to TileLoadQueueSpec.js.
Use context.createSampler to create samplers.

Previously, ImageryLayer was building the object literal manually.
Merge pull request #444 from AnalyticalGraphicsInc/useCreateSampler

Use context.createSampler to create samplers.
Add doc for TileLoadQueue.clear.
Merge pull request #443 from AnalyticalGraphicsInc/tileLoadQueue

Add clear method to TileLoadQueue.
Merge branch 'master' into background