Splitted E-Mail notification class into E-Mail notification and preference

Apart from this code has been modularized, fixed coding standards as well
as menu item name has been replaced as "Preferences"
Allow users to insert their signature in the threads

- User can specify their signature in Preferences submenu of SupportFlow
  menu
- There is also a checkbox in reply box which allow to prevent insertion of
  signature in a particular reply. This box is checked by default
- Signature will be added to reply in this format
  "$reply\n\n-----\n$agent_signature"
Using wp_kses() to sanitize agent signature
Merge pull request #144 from VarunAgw/preferences

Allow users to create a signature and automatically add it to replies.

The E-mail Notifications screen was refactored into a generic user preferences screen in order to accommodate this.