move markdown parsing to exporter

in addition to moving the markdown parsing to the exporter, this has the
following consequences:
 * more properties are being picked up on constructors
   eg dojo/Color now has blendColors, fromRgb, fromHex, etc
 * the return-description content is now being consistently parsed as
   markdown
 * parseMarkdown is probably being called more often than what would be ideal
   because the result is no longer being stored back on the metadata