Checking if post exists before accessing its properities

Currently if you view an non-existing thread e.g.
/wp-admin/post.php?post=25456&action=edit `$the_post` returns null causing
an error

So this commits check if it is null before continuing
Not returning wp_error on failed post creation

Returning `0` is good in case of failure in production use
Using wp_update_post() instead of using $wpdb to update post modified time
Authenticate SMTP settings when adding an E-Mail account

Fixes #140
Corrected coding standards
Sanitizing $_GET request parameter before using
Checking object class rather than its existence before accessing its
properties
Removed check for empty E-Mail account

We are only retrieving existing E-Mail accounts in get_email_accounts()
method so they is need to check for their existence again
Showing only E-Mail account username

Two different E-Mail accounts can't have same username so it is useless to
show IMAP host name along with it
Remove manual override of table columns width

There are causing table to take fix width even if there is lot of extra
space
For example in wp-admin/edit.php?post_type=sf_thread&page=sf_permissions,
status columns takes very small width even insufficient to shows it
content properly
Refactored code to make more readable
Making respondent E-Mail ID starts in same line as its photo in all
threads page

Currently if respondents E-Mail ID is long enough to not fit in line with
respondent photo it leaves the whole line empty moving to next line
completely
Show a dash sign if thread has no excerpt (i.e. no replies) in all thread page
Showing meaningfull message when there are no items in table

Tables cover:
- E-Mail accounts table
- User permissions
- E-Mail notification settings
Using a class rather than ID to get save button

Conflicts:
	classes/class-supportflow-admin.php
Improved messages shown when there are no items in table

This is update up to commit 17df98e65bc8a8e95302bacad25e1b8d50b7358f
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