Generate metadata for boolean defines in addition to exportable symbols
Rename generate-symbols.js to generate-info.js

This task generates build related metadata for the library based on doc annotations.  Since it is about more than writing out exportable symbols, it makes sense to have a more general name.
Add description to symbols and defines
Document boolean defines as positive assertions
Merge pull request #2039 from tschaub/defines

Add info on defines to general build metadata.
Load the icon when needed in ol.FeatureOverlay