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
Adjust border-radius of zoom buttons.
Merge pull request #150 from marcjansen/zoombtn-radius

Adjust border-radius of zoom buttons.
Add tile cache expiry infrastructure
Use option object rather than positional arguments for ol.animation.create* functions
Add ol.TileCache
Use ol.TileCache in ol.source.ImageTileSource
Use ol.TileCache in ol.source.DebugTileSource
Merge pull request #133 from fredj/options-obj

ol.animation.create* functions should use options objects rather than positional arguments
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
Care with transform

Since the transform method takes an arbitrary transform function, new coordinates may not be ordered in the same way as the originals.
Checked with third-party tool
Punctuated
Merge pull request #153 from tschaub/extent

Care with transform
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
Fix typo