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'
    ...

OK, I have checked this, its complicated :/   
Adobe started packaging the compiler with AIR 3.4 (uses flash player 11.4, released in aug. 2012). Versions before this do not include a compiler at all. 
This PR considers a project pure AS, if the Flex SDK is not added as a dependency. This means that if someone wants to make a pure actionscript project that uses FP < 11.4, need to use AIR >= 3.4 and add the `playerGlobal.swc` to `frameworks\libs\player\<version>`, and add the -playerVersion and the -swf-version compiler flags.    

Unrelated to this the code above is buggy, because it will use mxmlc-cli for AIR Flex projects (which as I know will fail to compile mxml), thus it should look like:     

    if (flexConvention.usesFlex()) {  
          taskName = "mxmlc"
    } else {
        taskName = "mxmlc-cli"
    }