Moodle 2.6 compatibility fixes

* Use core_useragent for IE check if available
* Add support for navigation_node::TYPE_SITE_ADMIN
* Edit buttons disabled (for now) on Moodle 2.6
* Switch from get_context_instance to context_XXX::instance everywhere
* Add (optional) nodetype param to decaf_expand_navigation::add_category
Version bump to 1.9.0 RC1