Allow proxy configuration settings to include protocol

http_proxy & https_proxy settings can now be specified with or without the protocol prefix. Fixes #538.

All these are now valid settings:

```ruby
  :http_proxy => 'http://username:pass@proxy.example.com',
  :http_proxy => 'username:pass@proxy.example.com',
  :https_proxy => 'https://username:pass@proxy.example.com',
  :https_proxy => 'username:pass@proxy.example.com'
```