Share books to e-mail

With this feature the user is able to share e-books to e-mail addresses. This is especially useful when using an Amazon Kindle. CalibrePHP will automatically detect which devices/e-mails are compatible with any of the formats and will show share button just for those addresses. Download/Share links are shown as drop-down menus.

![screenshot from 2013-06-11 20 52 30](https://f.cloud.github.com/assets/229567/638915/ad60b01c-d2c8-11e2-9440-5b979054c38e.png)

For this to work e-mail configuration has to be set up in `app/Config/email.php` and devices have to be defined in `app/Config/settings.php` like this:

```php
$config = array(
  'Settings' => array(
    // ...
    'share' => array(
      'example@kindle.com' => 'awz,mobi,epub',
      'example2@kindle.com' => 'awz,mobi',
    ),
  ),
);
```