Support strings as module identifiers

This adds support for strings module identifiers. It still supports the old ident modules but that will be removed eventually.

```js
module './a' { }
```

gets transformed to:

```js
$traceurModules.registerModule('./a', function() { ... }, global)
```

and

```js
import {x} from './a';
```

gets transformed into:

```js
var x = $traceurModules.getModuleInstanceByUrl('./a').x;
```

There is still a lot of work needed. Known issues:

- 'name' paths should be relative to the Project/WebPage
- export module m from './m' is broken. It needs to call getModuleInstanceByUrl outside the getter.
- Need to implement System.get
- Need to implement System.set
- Need to implement Loader normalization and resolve.

Review URL: https://github.com/google/traceur-compiler/pull/344

Closes #344.