adult content RTA label

[stevesouders]

To enhance the identification of adult content beyond the "2257" ( see #225 ) we could also look for an RTA label. See http://www.rtalabel.org/ .

RTA label appears to be less prevalent than 2257 based on surveying 18 adult websites, but the pattern is more reliable. Here are examples I found including the URL of the HAR file I searched:

http://httparchive.webpagetest.org/response_body.php?test=140501_0_1R&run=2&cached=0&request=2
<meta name="RATING" content="RTA-5042-1996-1400-1577-RTA" />

http://httparchive.webpagetest.org/response_body.php?test=140501_0_3Z3&run=3&cached=0&request=3
<a class="rta" href="http://www.rtalabel.org/" target="_blank"> </a>

http://httparchive.webpagetest.org/response_body.php?test=140501_0_5RE&run=2&cached=0&request=2
<meta name="rating" content="RTA-5042-1996-1400-1577-RTA"/>

http://httparchive.webpagetest.org/response_body.php?test=140501_5_32SW&run=1&cached=0&request=1
<a href="http://rtalabel.org/?content=validate&rating=RTA-5042-1996-1400-1577-RTA" target="_blank">
<img src="/rta.gif" alt="Verified RTA member"></a>

Based on this I recommend added a search for "RTA-5042-1996-1400-1577-RTA". If either this or 2257 is found then the site should be flagged as adult content.