AIR package added to as3corelib library

We have been discussing the need for an AIR specific version of the as3corelib library, which would contain useful APIs for working with Adobe AIR. However, instead of creating a new library, I have added an AIR package to as3corelib which will contain AIR specific APIs.

Right now, there is only one class added, a FileTarget for the Flex Log framework, but I expect we will be adding more over time.

If you have any suggestions, post them in the comments.