assertion message

I just made a small patch for #176. It supports assert statement with only one message expression and the type of the expression must be of type string. Other behaviors are almost the same as the issue.

```
class _Main {
	static function main (args : string[]) : void {
		assert false, "my message";
	}
}
```
```
$ bin/jsx --run /tmp/foo.jsx

/Users/nishiwaki.yuichi/workspace/JSX/.jsx.foo.jsx.89022.13f5c91287d.js:116
		throw new Error("[/tmp/foo.jsx:3:15] assertion failure: " + "my message" + "
		      ^
Error: [/tmp/foo.jsx:3:15] assertion failure: my message
        assert false, "my message";
               ^^^^^

    at Function._Main$main$AS (/Users/nishiwaki.yuichi/workspace/JSX/.jsx.foo.jsx.89022.13f5c91287d.js:116:9)
    at Object.JSX.runMain (/Users/nishiwaki.yuichi/workspace/JSX/.jsx.foo.jsx.89022.13f5c91287d.js:150:15)
    at /Users/nishiwaki.yuichi/workspace/JSX/.jsx.foo.jsx.89022.13f5c91287d.js:190:5
    at Object.<anonymous> (/Users/nishiwaki.yuichi/workspace/JSX/.jsx.foo.jsx.89022.13f5c91287d.js:191:3)
    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)
    at Module.require (module.js:362:17)
    at require (module.js:378:17)
```