Change state machine to include preauthorize state
Listing can be now preauthorized

- But not yet accepted
Preauthorized transaction can be now accepted
Several improvements
Start tests
Cucumber scenario for preauth
Automatically reject preauthorized request
Preauthorized to process hook
Fixing some tests
Add listing.payment_required_at?
Require payment details before posting
Fixed review comments
Merge pull request #467 from sharetribe/preauth

Preauthorize payment