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 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 #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
Add comment
Merge pull request #159 from elemoine/renderpath

Simplify map renderer
Merge pull request #154 from marcjansen/todo

Add fixme/todo target to build.py.
Test if browser supports Windows Pointer events
Direct all pointer events to js code (IE pointer events).

see: http://msdn.microsoft.com/en-us/library/ie/hh673557(v=vs.85).aspx
Merge pull request #157 from fredj/ms-touch

Direct all pointer events to js code (IE pointer events)
see: http://msdn.microsoft.com/en-us/library/ie/hh673557(v=vs.85).aspx
Add a comment about msTouchAction css on viewport
Add missing goog.events.EventType.DBLCLICK argument to unlisten
Remove event listener with goog.events.unlistenByKey
Don't remove event listener for this.handleUp_ twice.

The listener key is in this.dragListenerKeys_
Remove event listener with goog.events.unlistenByKey
Do not draw image if image is null