Outsource useragent detection

[ghost]

The browscap-js package uses the browscap database, which is the most exhaustive and active database of useragents I could find. It allows for easy useragent string parsing like so:

var Browscap = require('browscap-js');
var browscap = new Browscap();

var browser = browscap.getBrowser("Googlebot/2.1 (+http://www.google.com/bot.html)");

//Will log `true`
console.log(browser.Crawler);

Seems like a lot of the PR's for this repo are just adding bots, which is a duplication of the work done on browscap. Maybe it would be better to just use browscap-js under the hood and have this package be the interface for express.

[finppp]

This would be great, It's the reason I'm avoiding this package for now