[CORE] Move public resources to a /public directory

Advantages:
* Increases security by preventing direct access to file/
* We are careful and have a defined('GNUSOCIAL') || die() to prevent
  direct access to GS files, but we may miss one or a vendor/extlib may
  not be as careful
* Improves directory structure - It's more natural to physically
  separate what is public from what are GNU social resources

plugins/ShareNotice/css/README

@@ -1,9 +0,0 @@
-icon-sharing.png is from http://www.openshareicons.com/
-
-Shareaholic has made the Open Share Icon freely available for use by others under the
-Creative Commons Attribution-Share Alike 3.0 Unported License.
-
-
-icon-twitter.png is from http://twitter.com/favicon.ico and distributed under fair use
-icon-facebook.png is from http://facebook.com/favicon.ico and distributed under fair use
-icon-statusnet.png is from http://status.net/favicon.ico and distributed under fair use

plugins/ShareNotice/css/sharenotice.css

@@ -1,23 +0,0 @@
-.notice-share {
-    width: 24px;
-    float: right;
-}
-
-.notice-share li a {
-    display: block;
-    width: 16px;
-    height: 16px;
-    background: url(icon-share.png) no-repeat;
-}
-.notice-share li.notice-share-twitter a {
-    background-image: url(icon-twitter.png);
-}
-.notice-share li.notice-share-facebook a {
-    background-image: url(icon-facebook.png);
-}
-.notice-share li.notice-share-statusnet a {
-    background-image: url(icon-statusnet.png);
-}
-.notice-share li a span {
-    display: none;
-}