Refactor AJAX + tag button implemented

* Use `$.get()` jQuery method for conveniency.
* Implemented tag button in AJAX.
Fixed auth for `refresh.php`

Crontask can now be setup directly using a direct call to the script
like:
```
0 * * * * cd (...)/Freeder && php refresh.php >> logs/cron.log 2>&1
```
to update hourly (do not forget to create a `logs/` dir !).

TODO: Automatize the crontask adding.
Restore viewport + add explanations on webcron in settings
Hide buttons when anonymous view

_Note:_ I kept the "Share button" because it relies on external service
and can then be used even in anonymous mode but this can (SHOULD ?) be
debated.
Hide also the share button in anonymous view
Settings option to set the number of entries per page
Fix OPML import / add feed

Feed was not correctly refreshed after import / adding. cf issue #28.
Fix `refresh.php` for crontasks
Headers already sent in `refresh.php`
Fix bug in JS
Basic ugly read all button
Unread button
Start support of RDF
Trim the input from user, as indicated in https://github.com/FreederTeam/Freeder/issues/25#issuecomment-53045688
Automatic `.htaccess` generation

TODO: Check and install `.htaccess` at installation. Test everything.
Set `.htaccess` at installation time
Fix unread button + pagination
Progressive output for import / update

Started to handle it, need to add some JS to finish it.
Added some JS for progressive refresh output, have to debug it
Fix issue #34