Added methods for external attachments

- Method to add external attachment
- Method to check if is external attachment
- Method to retrieve external link to attachment
Added convince function to retrieve external attachment link
Added external attachment form html and styling
Added external attachment AJAX call back.
Moved add attachment functionality to wpba-functions.js
Added external attachment click handler method and attach attachment method
Added AJAX for external attachments add
Merge branch 'refs/heads/dev-1.3.8' into external-attachments

Conflicts:
	assets/js/wpba-media-handler-new.min.js
	assets/js/wpba-media-handler-old.min.js
Added JSDoc comments
Deprecated AJAX update_post_callback()
Added mime type select list
Updated gruntfile

Added:
- grunt-notify
- grunt-contrib-jshint
- grunt-contrib-csslint
- grunt-autoprefixer
- grunt-phpcs
Added PHP Code Sniffer requirement for composer
Added PHP Code Sniffer WordPress standard set
Added JSHint configuration file
Added CSSLint configuration file
Cleaned up JSHint issues
Cleaned up CSSLint issues
Optimized images
Ignored some composer specific files and reports