Added supprt for selecting wordpress users

I really didn't do much here.
Merge remote-tracking branch 'upstream/master'
Update support for user to use new RWMB_Base Class

update new field after a long overdue merge with the master repo
Merge pull request #409 from michaelhume/master

Add support for 'Users' Field
Added "category" in better include demo
Merge pull request #412 from jorpan/master

Added "category" in better include demo
typo
version
Merge pull request #414 from corvannoorloos/master

fix typo and update json version number
Update select2.css
Merge pull request #419 from krishna19/patch-1

fix height of select_advanced type
Fix bug in "text_list" field type

Fixed bug in "text_list" field type.
Merge pull request #421 from tonetlds/patch-1

Fix bug in "text_list" field type
Update image.php

Theme Check Error. Fix for Non-printable characters were found in the image.php file.
Update image-advanced.php

Theme Check Error. Fix for Non-printable characters were found in the image-advanced.php file.
Merge pull request #422 from krishna19/patch-2

Update image.php
Merge pull request #423 from krishna19/patch-3

Update image-advanced.php
Image sorting placeholder background color
Image sorting placeholder background color
Merge pull request #424 from sarojlakra/master

Image sorting placeholder background color