Refactor DOMHelper, introduce contextualElement

Template execution now requires both hooks and the dom to be passed in. For example:

```JavaScript
template(context, {hooks: hooks, dom: new DOMHelper()});
```

DOMHelper can be instantiated a few arguments:

```JavaScript
 // no contextual element (uses fallback div), global document, no namespace
new DOMHelper();

// contextual element, ownerDocument of element, namespace of element
new DOMHelper(document.createElement('table'));

// contextual element, ownerDocument of element, namespace provided
new DOMHelper(document.createElementNS(svgNamespace, 'foreignObject'), null, xhtmlNamespace);

// no contextual element (uses fallback div), document provided
new DOMHelper(null, document);
```

Optionally, you can provide a contextualElement to the DOMHelper. This will allow it to resolve un-escaped HTML rendering issues:

```JavaScript
var template = compile('{{{html}}}');
var tableElement = document.createElement('table');
template({html: '<tr><td>Yo</td></tr>', {
  hooks: hooks,
  dom: new DOMHelper(tableElement)
});
```

When block or component hooks are called in hydration, they are passed a DOMHelper instance with the current contextual element. This allows templates like:

```JavaScript
var template = compile('<table>{{#test}}{{{html}}}{{/if}}');
template({html: '<tr><td>Yo</td></tr>', {
  hooks: hooks,
  dom: new DOMHelper()
});
```

To run properly. The `test` helper just needs to pass the env (which contains the proper contextual element) through to the inner render.

This code also prepares htmlbars for changing namespaces based on the AST.
We don't need to stash the contextualElement on the AST anymore.