Fix issue with this and arguments in deferred functions

Fixes #425

Review URL: https://github.com/google/traceur-compiler/pull/432

Closes #432, closes #425.
Add support for import/export default

This adds support for

```js
export default 42;
```

and

```js
import x from 'm'
```

The default export gets exported using the name `default`.

Fixes #427

Review URL: https://github.com/google/traceur-compiler/pull/433

Closes #433, closes #427.
Change class transformer to use hoisting