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
Tidy up ol.source.MapGuide and add ratio option
Merge pull request #1339 from ahocevar/wfs-transaction

Transaction handling and exports for the WFS parser
Merge pull request #1341 from twpayne/mapguide-clean-ups

Tidy up ol.source.MapGuide and add ratio option
Safeguard against the case where no features are modifiable
Merge pull request #1344 from ahocevar/modify-remove

unable to removeInteration in modify-features example
Set the srs name when writing GetFeature

Also make clear that srsName is only optional for WFS 1.0.0.
Define type for applyWriteOptions call
Merge pull request #1346 from ahocevar/set-srsname

Set the srs name when writing GetFeature
Remove original handling on ol.Feature

This is another attempt to bring master closer to the
vector-api branch. In anticipation of the ability to keep track
of modifications on ol.Object through a beforechange event
(d7e4be0), we will be able to manage originals on the
application level or in a separate component outside of
ol.Feature.
Merge pull request #1347 from ahocevar/no-original

Remove original handling on ol.Feature
Add a way to delete vertices with the Modify interaction

After this change, vertices can be deleted by simply clicking on
them. This is the same behaviour as e.g. in geojson.io.