Add BinaryAttachmentResponseWriter, enhance ResponseWriters

Make all ResponseWriters extend AbstractResposneWriter.
Make ResponseWriters able to supply response headers and content length via GenericWritable in HTTPUtils.

This adds BinaryAttachmentResponseWriter to allow for the returning of downloadable binary responses. The IOOS build needs this to supply zipped netCDF responses. Previously I was using ServiceResponse, but this no longer seems possible now that all responses are pushed through AbstractServiceResponseWriter as AbstractServiceResponses. Using ServiceResponse was a bit of a hack anyway.

Comments or revision suggestions are of course welcome.