Asynchronous API for map#getFeatureInfo

This change allows us to use feature info services on the
server. It will also be useful for the GetFeature control to
get results from a WFS.
map#getFeatureInfo does not return anything

Thanks @elemoine for catching this.
Merge pull request #703 from ahocevar/getfeatureinfo-async

Asynchronous API for map#getFeatureInfo. r=@elemoine