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'
    ...

true, but in the case of a FlexUnit run we don't want the project linkage settings to influence the build of the FlexUnit swf. For example, a pure actionscript project which uses FlexUnit for its unit tests still needs to have the flex-config.xml loaded when building the FlexUnit swf, which would be reset if we didn't override the addFramework method here.
Maybe the name of this addFramework method is slightly misleading. It isn't really adding the framework, it actually handling the framework linkage. So 'handleFrameworkLinkage' is probably a better name. In case of a FlexUnit swf build, we wouldn't have to handle the framework linkage, we'd just use the defaults.