Steel 0.7.0

- Add shortcodes.php and move all shortcodes to this file
 - [x] Revise `column` shortcode using bootstrap code
 - [x] Add `tooltip` shortcode using bootstrap code (close #48 )
 - [x] Add `popover` shortcode using bootstrap code (close #49 )
 - [x] Add `carousel` shortcode using bootstrap code (close #51 )
- Add quotes feature (close #39 )
 - [x] Create custom post type with basic editor
 - [x] Create quote lists for various purposes
- Update everything to GPLv3
 - [x] Update `license.txt`
 - [x] Update disclaimer in `README.md`
 - [x] Update disclaimer in `steel.php`
- Admin styles
 - [x] Remove bootstrap styles from admin