Replace ol.overlay.Overlay with ol.AnchoredElement

This commit adds several features:
- rename of class to better reflect its behaviour
- full ol.Object support
- integration with postrender event for integration with animations and
  render loop
- minimised DOM modifications
- more precise positioning
- element placed in DIV to avoid modifications to user-supplied element
- correctly handle changes to the positioning property
Add tile cache expiry infrastructure
Add ol.TileCache
Use ol.TileCache in ol.source.ImageTileSource
Use ol.TileCache in ol.source.DebugTileSource
Add goog.structs.LinkedMap from Closure Library
Rename goog.structs.LinkedMap to ol.structs.LinkedMap
Add ol.structs.LinkedMap.prototype.peekLastKey
Expire old textures from texture cache
Port ol.TileCache to ol.structs.LinkedMap
Merge pull request #156 from elemoine/examplefix

Remove ref to userProjection
Merge pull request #151 from twpayne/tile-cache-expiry

Tile cache expiry
Use + rather than Number for string to number conversion
Merge pull request #152 from twpayne/texture-cache-expiry

Texture cache expiry
Merge pull request #148 from twpayne/anchored-elements

Replace ol.overlay.Overlay with ol.AnchoredElement
Add equals to Rectangle
Do not draw image if image is null
Add ol.SingleImageUrlFunction
Add ol.SingleImage class
Add ol.layer.SingleImageLayer