When selecting "Skip evaluation" priority is always reset to the default

This solves issue 97 (I hope).

When selecting "Skip evaluation" priority is always reset to the default priority on save, making it impossible to save the priority. This is because default_priority and fixed_priority saves to the same attribute. This commit "unmerges" the fields. 

I'm making a pull request because I'm unsure of the implications for renderings and making reports. The "Skip Evaluation" mode doesn't seem to be handled in most rendering...