node source map support

This is only a technology preview; experimental branch to demonstrate that it is possible to get source-map support in node environment.

```
$ cat /tmp/demo.jsx       
class _Main {
	static function main (args : string[]) : void {
		function foo () : void {
			function bar () : void {
				throw new Error('this is a demo');
			}
			bar();
		}
		foo();
	}
}
$ bin/jsx --enable-source-map --executable node --output demo.jsx.js /tmp/demo.jsx
$ node /tmp/demo.jsx.js

/tmp/demo.jsx:4
			function bar () : void {
  ^
Error: this is a demo
    at bar (/tmp/demo.jsx:4:3)
    at foo (/tmp/demo.jsx:3:2)
    at Function._Main$main$AS (/tmp/demo.jsx:9:5)
    at Object.JSX.runMain (/tmp/demo.jsx:9:5)
    at /tmp/demo.jsx:9:5
    at Object.<anonymous> (/tmp/demo.jsx:9:5)
    at Module._compile (module.js:449:26)
    at Object.Module._extensions..js (module.js:467:10)
    at Module.load (module.js:356:32)
    at Function.Module._load (module.js:312:12)
```