Add Koala.configure for custom urls/config.

To change the url that koala used to make requests....
```ruby
Koala::Facebook::GRAPH_SERVER = "some-host.mysite.com"
```

Now you can configure the koala url defaults
```ruby
Koala.configure do |config|
  config.graph_server = "some-host.mysite.com"
  # all subsequent requests will go to `some-host.mysite.com`
end
```

Additionally remove the constants that stored the various server urls...

:smiley: