Refactored Core Order Checkout

Guys,
To start into Spree codebase, I did a small refactoring at Order Checkout code. I removed some temp variables from methods and tried to make things more readable.

On block chaining, I didn't find any example to follow, so I followed this ![Ruby Style Guide - blocs](http://xaviershay.com/ruby-style-guide#blocks-and%20procs) - let me know if fits or have a better way. If I did not chained block with map, I would use do..end as well.

I hope this would be a kind of start :)