Fix bad include path error, remove deprecated wfLoadExtensionMessages() calls

These four commits fix three errors I encountered when installing this extension, the first and most breaking one is that the extension doesn't know where to look to find the twitteroauth.php library because the full path to the MediaWiki include directory is not defined.  This is fixed in this pull request.

In addition, one commit removes all calls to wfLoadExtensionMessages(), which was deprecated in MediaWiki 1.16 and has been removed as of MediaWiki 1.21rc4.  Although this breaks the extension's compatibility in MediaWiki 1.15 and earlier, these versions of MediaWiki are no longer supported, and not removing the calls to wfLoadExtensionMessages() breaks the extension in MediaWiki 1.21.

In addition, this pull requests adds checking if $ctAliases is empty before attempting to foreach it, suppressing many unnecessary PHP warnings and probably saving a tad bit of CPU time.