Update with AIR and ASC 2.0 support

This adds `<checkair> `and `<requireair>` macros. Existing `<compc>`,
`<mxmlc>`, `<amxmlc>`, `<packageair>`, and `<genairkeystore>` macros are updated
to use the ASC 2.0 compiler that ships with AIR, rather than the legacy
ASC 1 compiler from the Flex days.

For projects that use Flex components and cannot use ASC 2, support for
ASC 1 compilation is provided via the `<mxmlc-asc1>` and `<compc-asc1>` macros.
Projects that don't use Flex shouldn't use these macros.

Finally, the new ASC 2 compiler macros support the 'optimize' argument,
and default optimize to 'true' if not specified.