Add missing Express.js-style features

[nbeerbower]

New response methods:

• res.sendStatus(code) - send status code with text body
• res.append(name, value) - append to existing header
• res.location(url) - set Location header without redirect
• res.links(obj) - set Link header for pagination
• res.etag(value) - set ETag header
• res.lastModified(value) - set Last-Modified header
• res.locals - request-scoped variables

New request properties:

• req.hostname - parsed hostname from Host header
• req.subdomains - array of subdomains
• req.xhr - detect XMLHttpRequest
• req.fresh(etag, lastModified) - cache validation
• req.stale(etag, lastModified) - opposite of fresh

New app features:

• app.locals - application-level variables
• app.notFound(handler) - custom 404 handler
• app.param(name, handler) - parameter preprocessing
• body_limit option - configurable request body size limit

Also adds comprehensive tests for all new features.