Added Cookie#to_h
Element::Base: ONly symbolize 1st level option keys
Page#body: Defaults to ''
HTTP::{Message, Response}: Attrs converted to accessors
Added HTTP::Request#effective_cookies
HTTP::Message: Updated specs to match changes
HTTP::Client: Moved #queue to be public
Added HTTP::ProxyServer class

Moved the proxy server out of the `Proxy` plugin and updated it to work with
Arachni::HTTP objects.
Arachni::Browser: Real browser driver providing DOM/JS/AJAX support

[Issue #50]