Update class-coauthors-guest-authors.php

Should Meet WordPress coding Standards now. Adjusted, filter names to prefix with CAP-, also they are section & field specific. 

Test of the filter and adding a new field (checkbox)
https://gist.github.com/alpha1/9421107

limitations:
radio buttons likely do not work.
select/multi select will not work.
Textareas do not work. 

Only inputs that are similar to a test field, like email will work. Checkboxes also work.

I'll be happy to work towards supporting all of those in the future.