Update link-to to allow using in non-block form

This PR adds the ability to use `link-to` helper in non-block form. It checks if a block is not provided, and if so, uses the first argument passed as the link title. If the first argument is not a quoted string, the helper looks it up on the current context.

Below are some examples:

```handlebars
{{link-to 'About Page' 'about}}
```

```handlebars
{{#each posts}}
  {{link-to title 'posts.show' this}}
{{/each}}
```

I have been working on this with a lot of guidance from @machty (thanks!), and the following discourse topic describes almost all of the process: http://discuss.emberjs.com/t/help-contribute-to-ember-non-block-form-linkto/2039.