Update web.config to follow IIS 7.x restrictions

Fix #1536
See http://support.microsoft.com/kb/942076
Merge pull request #1542 from LeoColomb/iis-fix

Update web.config to follow IIS 7.x restrictions
No more tweetdeck. Fixes #1534
I want to be able to get all keywords for a specific URL.  While the function yourls_get_duplicate_keywords() exists, it returns NULL -- instead of a single keyword -- if yourls_allow_duplicate_longurls() is false.  This commit refactors the keyword logic from yourls_allow_duplicate_longurls() into yourls_get_keywords() so a keyword list can be obtained regardless of the state of YOURLS_UNIQUE_URLS.
Added sort (order by) options to new yours_get_keywords() function.
Clarified comment.
Moved filter into named function.
Fixed Travis error.
Updated function name. Deprecated old version. Updated yourls-infos.php.
Added filter to yourls_get_longurl_keywords. Fixed typo.
Fixed typo.
Created the function yourls_get_or_create_link() with enhanced keyword creation logic.
Refactored yourls_add_new_link to use yourls_get_or_create_link().
Moved filters to right location.
Bypassed filters on $return error.
Added code to selectively trigger an add_link... action for backwards compatibility.
Adjusted add_new_link logic to fully reproduce original output.
Ensured all add_new_hooks actions are wrapped with appropriate if clause.
Fixed typos.
Fixed typos.
Fixed returns.
Fixed typos.
Even more documentation.
Fixed typos to resolve Travis issue.
Fixed typos to resolve Travis issues.