Implement proper validation for the wpseo_taxonomy_meta option following the same philosophy as everywhere else in this branch

Including:
* upgrade routine to get rid of wrong values (including ''noindex' = 'on')
* refactored and checked everywhere the values are used
* refactored the update_term method
* deprecated wpseo_get_term_meta() function in favour of WPSEO_Taxonomy_Meta::get_term_meta()
* made WPSEO_Taxonomy_Meta::get_term_meta() more flexible, it used to return the meta value for an individual entry for a term in a taxonomy. Now  - depending on passed parameters - it can do that or return the full meta array for a term in a taxonomy. It also now accepts term_ids next to slugs and objects as input.

Also:
* Added W3_ObjectCache to autoloader
* Fixed: bug where W3TC cache for taxonomy meta data wouldn't always be refreshed when it should (wrong class exists conditional) and sometimes would when it shouldn't (would run even if update failed)
* Fixed: Bug where taxonomies which were on an individual level set to noindex and sitemap include 'auto-detect' would still be shown in the sitemap
* Fixed: bug in canonical urls where an essential part of the logic was skipped for singular posts/pages
* Fixed: breadcrumb title entry field would show for taxonomy even when breadcrumbs were not enabled

* added yet some more {} to various conditionals without {}