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

Texture cache expiry