Add MapGuide untiled map support to ol3.

This adds a new ol.source.MapGuide class that is initialized with an options object that can contain the following values:
 - projection: The projection of the Map Definition in EPSG format
 - url: The mapagent URL
 - useOverlay: Determines whether the GETMAPIMAGE (false) or GETDYNAMICMAPOVERLAYIMAGE (true) will be used for requesting the map image. When using GETMAPIMAGE, you must include a valid MAPDEFINITION parameter in the 'params' option property. When using GETDYNAMICMAPOVERLAYIMAGE, you must include a valid SESSION and MAPNAME parameters in the 'params' option property.
 - metersPerUnit: A required value used for calculating the map scale needed by the image request. This value can be calculated using the MapGuide Web API or obtained through the new CREATERUNTIMEMAP operation in MapGuide Open Source 2.6
 - params: A set of key-value pairs to append to the mapagent request
 - extent: The bounds of the layer

An example is included to demonstrate this new layer source. Like ol2, this example uses the MapGuide Server on data.mapguide.com
Merge pull request #1236 from jumpinjackie/master

MapGuide untiled map support