Sandbox

Add PagSeguro [sandbox](https://sandbox.pagseguro.uol.com.br/como-comecar.html) support.

In order to get PagSeguro's urls we can call `PagSeguro::Url.site_url("/some/path")` or `PagSeguro::Url.api_url("/some/path")` .

This methods returns the api or site url based on configured environment.

To change the environment:
```ruby
# eg: config/initializers/pagseguro.rb 
PagSeguro::Url.environment= :sandbox  #production by default
```

I update some gems and the rspec sintax.