HTTP endpoint for policy evaluation (execution)

This is an experiment to produce a generic evaluation function for different polices, which is able to return arbitrary JSON result after policy execution. For the moment a lot of conventions have to be followed in order for this to work, which is not so good. More experiments will follow to reduce the conventions to minimum (it would be best to remove all of them).

Current conventions that must be followed:

• policy package is always gaiax
• result variable is assigned with the expected result from the policy execution

Closes #12 (closed)

FYI: @tziai