Vsite favicon


#### Changes

* In `os.module`, add comments to existing favicon-related code.
* In `vsite_domain.module`, use `$form_state['build_info']['files']` to allow AJAX on the CP > Settings form.
* In `vsite` feature module, export 1 new field_base (managed file `field_favicon`), 3 new field_instance (1 instance of `field_favicon` per vsite group bundle), 1 new strongarm (`"theme_settings"`)
* Add update hook to revert the field_base and field_instance components for vsite
* Add update hook to set the new strongarm'ed variable (without reverting strongarm component)
* The new `"theme_setting"` variable value `toggle_favicon = 0` prevents `[includes/theme.inc#2500](https://github.com/drupal/drupal/blob/7.x/includes/theme.inc#L2500)` from invoking `drupal_add_html_head_link()`
* In `vsite_preprocess_html()`, we invoke `drupal_add_html_head_link()` if the vsite has a favicon value
* In `vsite_form_cp_settings_form_alter()` we add a file upload for the favicon field, and a submit handler to save the value to the vsite node.