Allow configuring via block and rspec tests

Closes #4 by allowing configuration via a block:

```ruby
Monk::Id.configure do |config|
  config.app_id = 'YOUR APP ID here'
  config.app_secret = 'YOUR APP SECRET here'
end
```

This is achieved by moving some of the existing configuration methodology into its own class 1c899f7 and then adding in the 


Additionally this pull request sets up the Rspec test suite. The suite was originally setup in  ```next-rspec``` where the existing (dev) version of configuration is tested. I recommend checking out that branch if you want to see the tests that do not have to do with these configuration changes. The rest of the commits are slight modifications to make testing work with configuration switch and testing the configuration features (c043ce2 & 31f9184).