Add {{link-to}} helper for HTMLBars.

This does seem to work (tested locally), but the link-to helper unit tests were SUPER sparse.  I added a few tests to confirm that escaping is handled right (seemed important to add some tests here).
Now that the arguments are always the same, we can rewrite this to be a little better.

```js
function deprecatedLinkToHelper(params, hash, options, env) {
  Ember.deprecate("The 'linkTo' view helper is deprecated in favor of 'link-to'");
  env.helpers['link-to'].call(this, params, hash, options, env);
}
```

Death to apply, death to `arguments`