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
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
Merge pull request #131 from VarunAgw/attachments

Add link to remove an attachment from a thread.
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