Async picking of rasterized features in imagery layers.

For example, by using the WMS GetFeatureInfo service.
Show position of selected feature.
If feature position is not known, use pick location.
Merge remote-tracking branch 'origin/master' into imageryLayerPicking
Add support for MapInfo GetFeatureInfo.
Add support for Esri WMS servers.
Add support for unknown GetFeatureInfo responses.
Give user control over GetFeatureInfo format.

Also add specs WebMapServiceImageryProvider.pickFeatures.
Add pickFeatures as an optional function on ImageryProvider.
Simplify layer feature picking and improve doc.
pickImageryLayerFeatures specs, and fix typo.
Add specs for Globe.pickImageryLayerFeatures.
Handle ServiceExceptions by reporting "No Features".
Update CHANGES.md.
Fix jshint warning.
Add WMS Sandcastle example.
Add WMS constructor option to disable pickFeatures.
Move pickImageryLayerFeatures to ImageryLayerCollection.
Revert unnecessary changes.
Improve pickImageryLayerFeatures test coverage.