Migrate payment processor docs from wiki

Created by: seancolsen

This PR migrates the following wiki pages:

• https://wiki.civicrm.org/confluence/display/CRMDOC/What+is+a+Payment+Processor
• https://wiki.civicrm.org/confluence/display/CRMDOC/Create+a+Payment-Processor+Extension
• https://wiki.civicrm.org/confluence/display/CRMDOC/Testing+Processor+Plugins
• https://wiki.civicrm.org/confluence/display/CRMDOC/Example+of+creating+a+payment+processor+extension

The content in these pages was very poorly organized. I tried to clean it up, and ended up moving quite a lot of stuff around in the process.

I would love to have someone like @eileenmcnaughton review this.

I'll merge it in one week (on 2017-08-26) if we don't get any reviews.