ActionScript checking via mxmlc

I noticed there was no syntax checker for ActionScript, so I quickly whipped this up based off of the jshint syntax checker.  It uses the mxmlc compiler and writes the compiled file to `/dev/null` on Unix systems and `NUL` on Windows.  This makes it a little slow on large projects, since it compiles any external classes along with the current file, but it works very well as far as I can tell.