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
Add fixme/todo target to build.py.

Will show a list of lines in sourcefiles that either contains the string TODO
or FIXME.

Example output:

    user@host:~/src/ol3 $ ./build.py fixme
      src/ol/source/xyzsource.js has 4 matches:
        #1         // FIXME add minZoom support
        #46        // FIXME use goog.nullFunction ?
        #59        // FIXME factor out common code
        #76        // FIXME we shouldn't need a typecast here

      src/ol/interaction/dragpaninteraction.js has 2 matches:
        #1         // FIXME works for View2D only
        #39        // FIXME works for View2D only
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
Make map handle view changes

With the frameState stuff the map renderer need not listen to view changes. The map renderer receives orders from the map. These orders are renderFrame_ calls with frameState objects representing the current state.
Make map handle size changes
Add comment
Make map handle background color changes
No more need for listener keys array