Don't instantiate provider in provider_class

It was possible for payment_class to be called, which instantiated a new
`PayPal::SDK::Merchant::API` object before paypal was configured by the
code in .provider. This caused the error:

  No such file or directory - config/paypal.yml

This changes provider_class to just return the class itself. It also
overrides `.provides?` to be more explicit about the behaviour.

Fixes #15
Fixes #26

Conflicts:
	app/models/spree/gateway/pay_pal_express.rb
Convert amounts like '21,99' to '21.99' because the Big International Payment Company can't handle international amounts

Fixes #11
Fixes #30
update rspec setup to support headless operation, primarily for CI automation
pull unit test fix changes, based on feedback from pull request to better_spree_paypal_express. includes a couple of bug fixes from original project as well.