Improve payment processor interfaces

CiviCRM payment processor types are tied into various form layer implementations, complicating the process of building processors - especially those which don't conform to the established button / form / notify types.

In implementation of a processor which differs from the existing types (PxFusion, where a form on CiviCRM site is submitted via direct POST to offsite processor, browser returned via token redirect), the process of handling the contribution and related actions (profile submission, event registration etc) is tied into form processing and not easily rewritten for the processor in question.

An ideal outcome for this would be if processors could -

1. Rewrite the output form entirely. (Generating a "clean" form for offsite submission is not as easy as it could be.)
2. Use CiviCRM API or BAO to trigger the various contribution validation and success calls, rather than relying on CiviCRM form processing to fire these.

Will look through the various processor methods and make some concrete suggestions shortly.