Add support for HTTP.auth chainable

As per https://github.com/tarcieri/http/issues/84, allows for shortcutting HTTP Basic Auth and OAuth 2.0 Bearer Auth requests.

Examples:

```ruby
HTTP
  .auth(:basic, user: "admin", pass: "admin")
  .get("http://website.com/path/to/restricted/area")
```

```ruby
HTTP
  .auth(:bearer, "[ACCESS TOKEN]")
  .get("https://website.com/path/to/restricted/area")
```

I'm not super-up-to-date on how bearer requests in OAuth work, and whether or not they need to be Base64 encoded. [Twitter](https://dev.twitter.com/docs/auth/application-only-auth) seems to think so, but [the spec](http://tools.ietf.org/html/rfc6750) doesn't seem to mention it anywhere.

Might be good to have a form like this?

```ruby
HTTP.auth(:bearer, token: "[TOKEN]", encode: true)
```