Add 'experimental-webgl' to ol.webgl.CONTEXT_IDS_ (IE11 support)

see: http://msdn.microsoft.com/en-us/library/ie/ff975238(v=vs.85).aspx
Remove webgl-experimental from ol.webgl.CONTEXT_IDS_
Merge pull request #1049 from fredj/webgl_ie11

Add 'experimental-webgl' to ol.webgl.CONTEXT_IDS_ (IE11 support)