Added is_https() function from CI 3 to application_helper

* Use is_https() in gravatar_link() function (also in
application_helper) and Assets library.
* application_helper's list_contexts() now allows the Contexts library
to determine required contexts and (optionally) filter contexts without
landing pages.
* new getContexts()/setContexts() methods in the Contexts library
(deprecated get_contexts()/set_contexts(), see docs/comments for
additional details)