Added if-modified-since header and updated url

https://github.com/4chan/4chan-API#api-rules requires the use of 'If-Modified-Since' in each request to their server.
http://www.ietf.org/rfc/rfc2616.txt section 4.2 states that http header field names are case insensitive so I added the header in allow lower-case.
* Added 'if-modified-since' header
* Updated URL to https://a.4cdn.org as per the API docs.