Updated docs for new methods and clarifications

- Updated docs for how to properly hook `validate_and_save` into
  WordPress
- Adds documentation for how `link` and `multiselect` fields work
New tests for validate_select()
Let's see if this coveralls things works
Properly adds phpcov with composer
Let's see if directory matters
Lets only generate the clover, not log it, too.
I guess we need a .coveralls.yml file?

If this doesn't work I'm giving up for now.
Adds an else condition to the array builder in `validate`

- Instead of wrapping the whole validation routine inside a check for
  whether the $_POST key was set, we add an else condition for what
  to do if all the other paths return false. This was preventing us
  from deleting the metadata if the field had been cleared.