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.
Show a dash sign if E-Mail account associated with ticket is deleted in all thread page
Corrected variable spelling

$email_accoun_id => $email_account_id
Load E-Mail account data only in new thread page

In "Details" meta box
E-Mail account dropdown is only shown in new thread page so these settings
should be load only in that case
Created SupportFlow_Email_Accounts->get_email_account($id) method

Return a single E-Mail account
Test if E-Mail account already exists for thread

This is a server side check. It prevent updating of post E-Mail account if it is already set
Prevent adding new replies to thread with associated E-Mail account deleted

Current approach will disable reply box, pre-defined replies combo, attach
file button,..It will still allow to change other settings There is also a
server side check for it when saving posts
Merge pull request #146 from VarunAgw/accounts

Disable threads when their associated e-mail account has been deleted.

Fixes #139