Add content type autodetection for .twig files
Merge branch 'bugfix/twig-html-content-type' into develop

Sets the autodetected content type for `.twig` files to `'text/html'`.

To override the default content type for a view, specify it explicitly:

**Site/routes.php:**

``` php
<?php
\HotMelt\Route::add('/^\/foo\.txt$/', 'MySite\\Site::foo', array('text/plain' ⇒ 'PlainText.twig'));
```

closes #6