Contact Form: Add Telephone Contact Form field

Related: #269
This line has `class='email'`, which I assume was carried over from the email code above it. Line  [1685](https://github.com/jeherve/jetpack/blob/da13fe2c79bb4826cbc04800a6cf972628ad6620/modules/contact-form/grunion-contact-form.php#L1685) has `"' class='" . esc_attr( $field_type ) . "`, which seems like it would set a more appropriate class here (presumably, 'telephone').