Add GetFeatureInfo format

This PR proposes to add a format reader for WMS GetFeatureInfo request.

The format is able to read WMS GetFeatureInfo responses that starts with `<msGMLOutput>` or `<FeatureCollection>`.

It is based on `ol.format.GML2` format to read the features.
You can add a `@type` to this variable, to make sure you do not insert something else than `ol.Feature` objects into the array.