* `get_http_url()` renamed to `get_http_response()`.
* Use the same HTTP context to retrieve response headers and content.
* Follow HTTP 301 and 302 redirections to retrieve the title (default max 3 redirections).
* Add `LinkUtils` to extract titles and charset.
* Try to retrieve charset from HTTP headers first (new), then HTML content.
* Use mb_string to re-encode title if necessary.
Relates to #314 & #326
Additions:
- add global `cleanup_url()` and `get_url_scheme()` functions
Modifications:
- replace `Url` usage in `index.php` by calls to global functions
- fix `Url` tests not being run: PHPUnit expects a single test class per file
- move classes to separate files
