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.
If you `@type` the `layerFeatures` variable you may not need a type cast here – the `goog.array.extend` function uses `@template`.