Can use ASC 2.0 for AIR projects; Can make AIR libraries; Framework linkage fixes

This is a bigger commit than my previous ones, but adds more stuff too :)
Main changes:   
- Pure AS AIR apps/lbraries use the ActionScript 2.0 compiler. (maybe web projects would compile too if they don't have mxml)
- It is possible now to make .swc files that have AIR code
- Pure ActionScript AIR projects don't need the Flex framework as dependency
- The framework linkage logic was quite buggy for non-Flex projects, now it should work well

I'm making some comments in the code to make it more clear.

You can add the AIR SDK + Compiler dependency quite easily:   

     ivy {
            name 'Adobe Air SDK'
            artifactPattern Os.isFamily(Os.FAMILY_WINDOWS) ?
                'http://airdownload.adobe.com/air/win/download/[revision]/[module]_Compiler.[ext]' :
                'http://airdownload.adobe.com/air/mac/download/[revision]/[module]_Compiler.[ext]'
        }
    }
    dependencies {
        airSDK group: 'com.adobe', name: 'AIRSDK', version: '15.0', ext: Os.isFamily(Os.FAMILY_WINDOWS) ? 'zip' : 'tbz2'
    ...

mxmlc-cli is only available in the latest AIR SDKs, so this would break for projects with an older version of the AIR SDK. I would only use the mxmlc-cli if it's an AIR project AND it doesn't use Flex, in which case it's a pure actionscript project with one of the newer AIR SDKs