Created a button to remove attachment from thread

Earlier there was no button/link to remove an attachment. So if you
inserted an attachment you have to refresh the page to remove it

Fixes #36
Corrected WP coding standard
Escaping attachment URL and filename
Making attachement remove link red in color
Escaping attachment ID before insertion in DOM
Merge pull request #131 from VarunAgw/attachments

Add link to remove an attachment from a thread.
Merge pull request #143 from VarunAgw/minor-changes

Minor refactoring and cleaning up.
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.
Embed "Forward this conversation" input box in a form tag

Currently after entering E-Mail ID's in forward conversation input box if
you hit enter thread get updated rather than sending of E-Mail. So this
empty form tag is just to redirect enter to this send E-Mail button