[himura467]

Closes #16

• Adds Nack::Request, which wraps any object satisfying SPEC.md section 3 and reads each field lazily, copying a buffer into a frozen String only on first access.
• Makes request_method, script_name, path_info, query_string, headers and body writable. A nil path or query reads as "", and * stays "*".
• GET, POST and cookies are cached against the exact object they were parsed from, so replacing query_string, body or a header reparses them. params merges GET and POST on each call.
• POST reads only an application/x-www-form-urlencoded body, chunk by chunk through body.read, and leaves any other body unread.
• cookies returns values as sent, without percent-decoding, keeps the first value of a repeated name (RFC 6265 §5.4), and reads a pair without = as a cookie with an empty name (RFC 6265bis).
• host and port come from authority only, with port falling back to the default for scheme. A malformed authority or a port above 65535 yields nil for both.
• Names the peer accessor remote_address instead of the ip the issue asks for: it returns the transport peer as SPEC.md defines it, including the unix: form, not a client IP behind proxies as Rack's ip does.
• Passes connection, protocol, authority, content_length, variable, trailers, inform and tunnel through from the bridge, and adds context.
• Does not parse multipart bodies.