Add out-of-the-box support for date formats used by HTTP/1.1

Adds support for RFC 1123, RFC 850, and ANSI C asctime() date formats
Details: http://www.w3.org/Protocols/rfc2616/rfc2616-sec3.html#sec3.3.1

The primary use case for this is for RestKit applications to be able to set a mapping from `@"@metadata.HTTP.response.headers.date"` to an `NSDate` instance. This doesn't currently work out of the box because the date formats supported by the HTTP protocol (see link above) aren't among the default value transformers.