Add support to OAuth token authorization in Highrise

As `ActiveResource::Base` headers aren't inherited by subclasses, this patch was needed in order to support OAuth 2.0 token authorization with Highrise.

With this it becomes possible to use this gem for the scenario asked in #15, by doing:

```
$LOAD_PATH.unshift File.dirname(__FILE__) + '/../lib'
require 'highrise'
require 'pp'

Highrise::Base.site = 'https://example.highrisehq.com'
#Highrise::Base.user = 'xxx'
Highrise::Base.oauth_token = 'YOUR TOKEN HERE'

@tags = Highrise::Tag.find(:all)

pp @tags
```