4xx/5xx errors with their statusCode in it.

### Usage

``` js
var BadRequestError = require('./http-errors').BadRequestError;

function doSomethingBad() {
  throw new BadRequestError('It went bad!');
}

try {
  doSomethingBad();
} catch (err) {
  assert.strictEqual(err.statusCode, 400);
}

```

### Features

* **4xx/5xx status as errors classes** - Status text is the class name, e.g. UnauthorizedError, NotFoundError
* **Status code property** - Error handlers can use the status code to send responses.
* **Correct stack trace** - no extra stack frames, no double capturing of the stack trace

### Anti-patterns

These are some things that I've seen in other proposed solutions that you should avoid.

* `Error.call(this)` - creates another error object (wasting a bunch of time) and doesn't touch `this` at all
* `Error.captureStackTrace(this, arguments.callee);` - works, but `arguments.callee` is deprecated, so don't use it
* `this.stack = (new Error).stack` - this... I don't even...