update app descriptor to AIR 4.0
Fix tests
README: fix building instructions

The runtime is built with AIR; the exporter is built with Flex
README: how to build the demo
README: links to pre-built exporter & runtime
Optional pretty printing.
Teach the UI about the new parameter
Fix XML pretty printing.

Totally read the API docs wrong and missed that XML.prettyPrinting is
static, so I just added that to the document instead of changing things.