New features patch

I have made a few changes:

- Ability to choose destination for link (options are current page (default), front page or custom page.
- Added class to thank you message div to match theme status message (class="messages status").
- Ability to turn off JavaScript.
- Ability to modify thank you message.
- Name input field added. Can be used in the email as !name.
- Removed unnecessary calls to variable_set. Using system_settings_form is enough.
- Added spread_add_files() so files can be added via the theme layer. A static variable is used to ensure files are only added once.
- Added proper labels to the friend's email fields. This should be hidden using CSS on the theme layer if required.
- Changed the validation a bit.

Hope you like the changes.