Add userAgent to URLRequest

This pull request adds userAgent to URLRequest for all backends:
* Native: The userAgent is used by the CURL implementation of load()
* Flash: The userAgent get/set is a noop if it doesn't exist.
* HTML5: The userAgent field is ignored.

This pull request also adds Lib.notImplemented to the flash backed.  This was done by replacing the openfl.Lib typedef with a wrapper class.