Refactor lib/stripe.rb

This was the first file I opened when auditing this code. IMO it needed some refactoring.

Summary of changes:
- Break apart monstrous `Stripe.request` method. It does way too much.
- Reduce cruft
- Sensible max line length for those working with the code in an editor with multiple splits, e.g. VIM

I wanted to go further with breaking up `Stripe.request` and move the RestClient exception handling into `Stripe.execute_request` but due to the way the client is mocked in testing it broke the tests despite being a simple refactor. I would have continued but it's late. I promise to go back to it if you accept these changes.

No behaviour has been changed. Tests are green.

Thanks!

S