Adding a verification criterion causes a 500 error.

Inserting any form of verification criterion (with just a 'kind' key, no other keys) causes a 500 error and this on the console:

```
  File "/home/jimmacarthur/trustable/mustard/mustard/elementtree.py", line 202, in _resolve_mapped_here
    self.elements[ref].mapped_to[path] = element
AttributeError: 'VerificationCriterion' object has no attribute 'mapped_to'
```
