2 more bugfixes for custom sidebar support.
Delete sidebar transient on upgrade to force a fresh query
Address some 'variable not defined' issues.
Fix sb_pings callback to use returnable function.
closes #48
Filters for overriding StartBox images.
Modify post image filter names
Featured Content widget tweaks:
- sanity check on posts_per_page number
- no_found_rows because we're not paginating and saves query time
- post_status: extra failsafe to prevent non-published content
- ignore_sticky_posts: because we just want to return the most recent and it adds +1 to the post number returned
Add filter to Featured Content widget that allows overriding of the posts returned (copied from the Recent Posts widget in WP core)
example `if ( $args['widget_id] == featured-content-widget-2 ) $query_args['post_type'] = 'my-post-type';`
add_meta_box shouldn't be called directly from the admin_init hook since the $screen object isn't available yet.

Moved the add_meta_box to the load-$this->page hook.

This solves the issue of the error log being overrun with "Trying to get property of non-object in /wp-admin/includes/template.php on line 822" notices.
remove unnecessary global $post call
the_post() calls should be preceded by have_posts() checks
closes #54
Enable filtering on featured content instance
Use WP functions. wp_get_nav_menus() and selected()
update menu picker to use wp_get_nav_menus()

This is so all menus are displayed in dropdown selects and not just menus with items added to them.
Variable sanity
Some alignment OCD
Filter edits for sb_post_image
Remove redundant $wp_query global declaration

Fixes #46
First pass at widget code improvements:

* New widgets.php file introduced which contains the code that causes
effects (registers widgets), leaving the existing files to be renamed as
pure class files.
* Redundant widget registration functions formally deprecated.
* Widget setting defaults consistently moved into constructor for easier
identification.
* Fixed un-internationalized strings.
* More adherence to code standards.
* Some double-quoted strings changed to single-quoted strings.
* Cleaner and moe consistent coding style, making code easier to read.
* Files, classes, methods and functions all documented.
* Type-hinting added for array parameters.
* More appropriate sanitization for numerical fields.
* Escaping added for some attributes.
* Use of selected() where appropriate.
* A few todo comments left as reminders for later passes of the code.
* Version added to enqueued resource so cache can be automaticalled busted.
* Removed PHP4 constructor naming.
Rename widget files back to original