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.