Modified page_wrappers_preprocess_page() logic so that Context can set page wrappers on non-node pages.
Updated _page_wrappers_tokens() so [node:*] tokens won't cause PHP errors on non-node pages.
Merging pull request 476.

Merge branch '7.x-4.x-page-wrappers-for-non-node-pages' into 7.x-4.2.6