Fixed saving of email for guest user while checkout #4442

Updated test for the same
Merge pull request #28 from tarunbhardwaj/develop

Fixed saving of email for guest user while checkout #4442
Move out actual payment processing from payment_method #4451

:gift: Moved actual payment processing code out of payment_method to
_process_payment method. So that other modules can easily modify
the behavior of processing payment independently.
Added coverage, coveralls and readme.rst
Merge pull request #30 from tarunbhardwaj/develop

Move out actual payment processing from payment_method #4451
Fixed guest access code for sale #4442

:beetle: Guest access code was created after sending email(confirming) for sale.
Infact access code should be created before sending email so that link
in email should also contain access_code for guest user.