Adds Shipping Cost to the generated checkout XML

Before this change, it wasn't possible to define a global shipping cost.
PagSeguro allow us to define a global one so you don't have to define
one for each item in the cart.

This commit includes this change as well as its tests.
Melhorando o exemplo de recebimento de notificação

Melhorando o exemplo de recebimento de notificação
Merge pull request #6 from jacksonpires/patch-1

Melhorando o exemplo de recebimento de notificação
Merge pull request #5 from kurko/add_shipping_cost

Adds Shipping Cost to the generated checkout XML
bumping version to 0.4.1
increasing test coverage and refactoring specs
adding validations to PreApproval attributes
further refactoring code adding factories, and tweaking pre approval fields based on trial and error
updating readme to document recurring payment
adding .sublime-project to .gitignore
added find method to query
format query custom date interval
adding specs to PagSeguro::Query#find
bumping version to 0.5.1
adding abandoned option to PagSeguro::Query#find, which allows to fetch abandoned transactions
adding gotcha information on README
removing unused assigned variable
fixes exoteric error #10 thanks to @josuetex
Improved core readability for split screen (~80 characters/line).
Removed unwanted whitespaces.
Removed unwanted whitespaces from specs.