PP-ct Validation - validate payments before they are approved

[eileenmcnaughton]

Currently the user is required to validate payment with paypal again if they make an error in the form

Options to get past this are

1. jquery validate before launching the paypal console would be the most user friendly option -( ie. don't launch paypal until we have valid input) I think we could conditionally enable the script based on some value being assigned from the tpl to the template - we DO seem to need to call Validate from the template. We'd also need to make it respond to 'deeper' validate calls - ie. calling ContributionPage.validate api

Currently the user message is a bit fugly too :(

2. suppress the paypal button after it has been used & just hold the token somewhere on the form & have them re-submit. This might be easier & perhaps required as well for now - but the validation affects more than just paypal.

[eileenmcnaughton]

Note this PR would improve the validation options #98