Massive commit

Massive commit
- renamed index_ajax.php to admin-ajax.php: name more accurate, no underscore to avoid error with htaccess RewriteCond (wtf?)
- renamed all parameters "mode" to "action"
- added nonces to Add, Edit, Save, Delete buttons. Fixes issue 425.
- changed behavior & args of yourls_verify_nonce()
- cosmetic tweak in CSS (edit row bg color)
- new function: yourls_site_url() to return YOURLS_SITE with http(s) preference
- comply to SSL pref when loading ressources. Fixes issue 512.
- Ajax URL now echoed in head, complying to SSL pref

git-svn-id: http://yourls.googlecode.com/svn/trunk@511 12232710-3e20-11de-b438-597f59cd7555