getFeatures method and featureInfo templates

To avoid surprises for application developers, this change
creates a new getFeatures method. So it is clear now beforehand
whether features or feature info markup is returned. The result
is now also grouped by layer, so application developers always
have a link between a layer and the feature info it returns.

To make getFeatureInfo return markup for vector layers, this
change also adds a featureInfoFunction property to the vector
layer, which gives developers full control over how features are
rendered to feature info markup.
Renaming featureInfoFunction to transformFeatureInfo
API doc improvements
Merge pull request #744 from ahocevar/getfeature

getFeatures method and featureInfo templates. r=@bartvde