Add basic layers support
Add support to add/remove layers and change their levels.
Add layer-level-changed event and add $.indexOf polyfill
Fix $.indexOf pollyfill
Add the original options in addLayer events
Add layers demo page
Uncomment version
Add viewer.getNumberOfLayers method
layers.html test file:
-display 2 layers after opening
-fix order
-add opacity demo
-add rotation demo
Add viewer.getLayerAtLevel method and rename getLayerLevel to getLevelOfLayer.
Add layers unit test.
Prevent adding collections as layers.
Throw an error when trying to add a layer in collection mode.
Allow base layer reassignment if not in sequence mode.
Support removal of base layer.
Add layersAspectRatioEpsilon check
Update the source when the base layer change.
Merge branch 'master' into layers

Conflicts:
	src/drawer.js
	src/viewer.js
	test/test.html
Separate layers and overlays to ensure that overlays are always on top of layers.
add crossOriginPolicy drawer configuration to enable or disable CORS image requests
Add changelog for #298
Merge pull request #298 from avandecreme/layers

Layers support
Minor tweaks to changelog
Merge pull request #364 from sul-dlss/cors-policy

add crossOriginPolicy drawer configuration to enable or disable CORS image requests