Render error tiles as empty
Treat image tiles with a zero dimension as empty
Return listener key from ol.Object#on
Add ol.Object#unByKey
Add ol.Object#onOnce
Only set center once in mobile full screen example
Optimize canvas renderer.

Only use setTransform when it's needed (ie. when the view is rotated)
Merge pull request #662 from fredj/canvas-draw-image-perf

Possible canvas performance enhancement
Remove unused CSS classes

The classes ol-layers, ol-layer-image, and ol-layer-tile are not used.
Merge pull request #658 from twpayne/ol-object-events

ol.Object events
Merge pull request #663 from twpayne/render-error-tiles-as-empty

Render error tiles as empty
Strip debug classes when compiling
Add spiral to Madrid animation example
Cosmetic clean-ups
Merge pull request #669 from twpayne/more-animations

Minor changes to the animation example
Don't escape HTML characters in shader generation

This tells Mustache not to escape <, >, &, and " characters.
Merge pull request #670 from twpayne/fix-glsl-escaping

Don't escape HTML characters in shader generation
Re-enable logging
Log WebGL shader and program errors
Refactor tile functions

* Add support for minZoom in XYZ tile grids and tile sources
* Factor out common tile coordinate transforms