Usability update & bugfixes for meta tag description check

Usability update & bugfixes for meta tag description check. Code tested in both WP 3.5.2 as well as WP 3.6RC1.

* Bugfixes
	* Fixed: Meta description tag discovery looked in parent theme header file even when a child theme is the current theme.
	* Fixed: Using the 'Fix it' button would remove the meta description tag from the parent theme header file, even when a child theme is the current theme.
	* Fixed repeated unnecessary meta description tag checks on each visit to dashboard page.
	* Fixed: Using the 'Fix it' button would fail if it had already been used once (i.e. if a wpseo backup file already existed).
	* Fixed: Meta description 'Fix it' feedback message was not shown.

* Enhancements
	* Added checks for the meta description tag on theme switch, on theme update and on (re-)activation of the WP SEO plugin including a visual warning if the check would warrant it.
	* Added the ability to request re-checking a theme for the meta description tag. Useful when you've manually removed it  and want to get rid of the warning as inspired by [tzeldin88](http://wordpress.org/support/topic/plugin-wordpress-seo-by-yoast-your-theme-contains-a-meta-description-which-blocks-wordpress-seo).


N.B.: the check on theme update is only run when the theme is updated using the WP update functions. Manual updating of themes is currently not detected.
(Could be done with an option which remembers the last version checked, but might be a bit overkill)

N.B.: the .pot file will need updating when this change is merged as one new phrase has been added.

This fixes a number of forum threads, amongst which:
http://wordpress.org/support/topic/plugin-wordpress-seo-by-yoast-delete-meta-description
http://wordpress.org/support/topic/plugin-wordpress-seo-by-yoast-warning-in-dashboard-after-install-yoast-1287-about-metatags
http://wordpress.org/support/topic/plugin-wordpress-seo-by-yoast-yoast-meta-description-1
http://wordpress.org/support/topic/remove-warningerror-after-fixed