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), which prevented the removal of meta description tags in upgraded theme header files.
	* 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).