Remove Notices

[Sephster]

This pull request aims to achieve a number of things, most of which have been listed in the updated readme:

• Change WP_Widget to call __construct() instead of old PHP 4 constructors
• Implement multiple isset() checks to ensure we don't access unidentified indexes
• Add support to the widget for the WordPress customiser
• Update code format to match some WordPress core conventions
• Change openattribute_widget.php to class-openattribute-widget.php
• Change plugin folder name to be openattribute instead of openattribute-for-wordpress, matching the main plugin file as per WordPress recommended naming conventions

I have tried to test this as much as possible but would appreciate a second pair of eyes to check if there are any glaring errors, as there are quite a few changes here.

One of the biggest ones is that the plugin folder should now be listed as openattribute instead of openattribute-for-wordpress to more closely match recommended WP naming conventions.

If there are any concerns with this plugin, please let me know and I will endeavour to rectify these as quickly as possible.