Add support for temporal conditions in rules

[dfunckt]

This is useful to determine staleness of a record. Here's a contrived example:

Term: status
  Concept Type: Short Text (Type)
Term: update timestamp
  Concept Type: Date Time (Type)

Term: operation
Fact type: operation has status
  Necessity: each operation has exactly one status.
Fact type: operation has update timestamp
  Necessity: each operation has exactly one update timestamp.

Rule: It is necessary that each operation that has a status that is equal to "success", has an update timestamp that is at most 5 minutes ago.

Ref: https://www.flowdock.com/app/rulemotion/platform/threads/xm_QlDi3QQRDdoo3qcwOZ8bEZeE