Support for writeHead method

This adds `res.writeHead(code, reason, headers)` support, handling both optional reason and headers.  It does not support the deprecated array form for headers.  The response `statusMessage` is set when the `statusCode` is set.  Values come from the `http.STATUS_CODES` lookup and can be customized by providing a reason argument to `res.writeHead()`.
Merge pull request #1 from tschaub/write-head

Support for writeHead method