Add the http proxy setting

My web server is in my internal network. I need to set the http proxy settings.
Add the http proxy setting.

Basic
```ruby
keen = Keen::Client.new(:project_id => 'XXXX', :write_key => 'XXXX', :read_key => 'XXXX', :master_key => 'XXXX')
```

Proxy
```ruby
keen = Keen::Client.new(:project_id => 'XXXX', :write_key => 'XXXX', :read_key => 'XXXX', :master_key => 'XXXX', :proxy_url => 'http://proxy.example.com')
```

Thank you!