Feature ant refactor

Hi,

I've refactored the ANT build script:
* added comments and the license header
* modularized build targets
* improved compatibility with Adobe AIR SDK

The build script now checks for the FLEX_HOME and AIR_HOME environment variables or uses the previous default one (shipped with Flash Builder).

I've tested the build script using Adobe AIR 14.0 and Apache Flex 4.13.

Hopefully this will enable successful builds using Travis CI service, if merged together.

Best Regards!