Remove data: from allowed protocols

Too many problems: too long URLs causing "413 Request Entity Too Large", browsers not liking to be redirected to a data URI, etc...
README update

Was still mentioning 2000 char limit on long URLs. See  https://twitter.com/hidgw/statuses/323347891362070528
Remove notice that has probably been here for decades.

<b>Warning</b>:  Illegal string offset '31.8888888888' in <b>D:\yourls\includes\functions-formatting.php</b> on line <b>21</b>
Stuff related to public shortening[1] & user auth[2]

1- readme note and linkage to wiki
1- fix global $yourls_allowed_protocol to allow plugin override
2- rewording of phpdoc comment
2- action added to yourls_maybe_require_auth()
Changelog update for YOURLS 1.6
GeoIP update
This is it: version 1.6
Number of hits displayed twice twice.
Update CHANGELOG.md
News in README
Fix twice </strong>

Fix #1346