Allows HTTP headers to be represented in a structured way. The parsers for each header field are represented as separate classes implementing the 'com.mattunderscore.http.headers.HeaderParser' interface. The values of each header field are represented as separate classes implementing the 'com.mattunderscore.http.headers.HeaderFieldElement' interface. Where a header field has a list of values each of these values is represented as a separate object of the same class. When these values are qualified the class implements the 'com.mattunderscore.http.headers.Qualified' interface.