Default to site default role

[andrewlimaza]

• ENHANCEMENT: Added the ability to create the pmprorole<> when we detect the level doesn't have this role created and not only on initial level creation.
• ENHANCEMENT: If no role is selected for a level, it automatically selects the sites default user role and doesn't create the custom role anymore.

All Submissions:

• [x] Have you followed the Contributing guideline?
• [x] Does your code follow the WordPress' coding standards?
• [x] Have you checked to ensure there aren't other open Pull Requests for the same update/change?

[andrewlimaza]

Resolves: https://github.com/strangerstudios/pmpro-roles/issues/31

[andrewlimaza]

This also resolves some issues with PMPro 3.0+