URI Routing overhaul

 - Allow multiple levels of controller directories (supersedes PRs #390, #2439)
 - Add support for per-directory 'defaul_controller' and '404_override' (resolves issue #2611; supersedes PR #939)
 - Fixed a bug where default_controller was called instead of triggering 404 if the current route is inside a directory
 - Removed a few calls from CI_Router to CI_URI that made a necessity for otherwise internal CI_URI methods to be public:

    - Removed CI_URI::_fetch_uri_string() and moved its logic into CI_URI::__construct()
    - Removed CI_URI::_remove_url_suffix, CI_URI::_explode_segments() and moved their logic into CI_URI::_set_uri_string()
    - Removed CI_URI::_reindex_segments() altogether ( doesn't need further manipulation, while  is
      public anyway and can be properly (and more effectively) replaced on the spot)
Merge branch 'feature/uri_routing_overhaul' into 'develop'