Support for ES6 - Harmony

I started to implement the support for ES6 and got quite far, yet the compilation is still failing on a following error: `Error loading app files Object #<Object> has no method 'checkObjectCoercible'` 

I do not have much more time this weekend so I may check it back on Monday. I am doing this pull request only if you need these files. I checked also how the other package is compiling files and it is in following manner:

```javascript
var traceur = Npm.require('traceur');

Plugin.registerSourceHandler("next.js", function (compileStep) {
  var oldPath = compileStep.inputPath;
  var newPath = oldPath.replace(/next(?=\.js$)/, 'now');

  var source = compileStep.read().toString('utf8');
  // force Traceur to define `this` in function scope
  source = "this;\n" + source;

  var compiler = new traceur.NodeCompiler({
    sourceMaps: true
  });

  try {
    var output = compiler(source, oldPath, newPath);
    compileStep.addJavaScript({
      sourcePath: oldPath,
      path: newPath,
      data: output,
      sourceMap: compiler.getSourceMap()
    });
  } catch (err) {
    compileStep.error({
      message: err.message,
    });
  }
});
```

I tried to use this approach as well but to me following line return an object `compiler` and not a function. Not sure what's going on.

```javascript
  var compiler = new traceur.NodeCompiler({
    sourceMaps: true
  });

// compiler is object not a function so I could not do:
compiler(content) // fails with #Object is not a function
```

I hope I helped a bit. If I have time on Monday I will continue.