Iterate on WorldPay implementation

[JustinSainton]

Stuff that needs done:

• Make sure it works with TEv1 and TEv2
• Make sure tokenization (javascript) is working in both TEv1 and TEv2. TEv1 works ATM
• Related to above make sure token creation works in both TE with proper field names sent in the creation request and in case creation fails do something perhaps ?
• Error handling for requests in case they fail for whatever reason
• Implement recurring billing (including supports(), metabox abstraction, and logic abstraction so we aren't dependent on always doing it the way Memberships currently does it)
• Abstract logging function out of WP/Amazon into Payment_Gateways
• Use jquery.payment from Stripe for default forms.
• Documentation (File headings, Method/Function doc blocks, filters)
• Consult prior art for other potential improvements to abstract

[JustinSainton]

For 4.0, we'll focus on the first three issues - specifically ensuring that this works as expected in TEv2.

[JustinSainton]

@misulicus I think we're all good here now on those top three issues? Confirm and let me know - if not, we can move back to 4.0