Add a fcshctl_mxmlc syntax checker for actionscript

Fcshctl allow mxmlc to run way faster (http://hasseg.org/blog/post/194/fcshctl-the-flex-compiler-shell-controller/). This syntax checker takes advantage of it. In order to make it work, one must install fcshctl and install, in the same folder, this script: https://github.com/lettertwo/as3CompilerErrors-tmplugin/blob/master/AS3Errors/resources/scripts/fcshctl-mxmlc This script is needed because syntastic doesn't allow spaces in the checker name, therefore "fcshctl mxmlc" don't work but "fcshctl_mxmlc" does.