Refactor policy evaluation response to allow different levels of embedding values

Closes #33

See merge request !24

This commit changes two things:
* evaluationID is no longer returned as part of the JSON response, but instead is
returned as HTTP ETag header
* if policy source code contains a single blank variable assignment then the result
from the policy evaluation will be only the value of that variable

Revert MongoDB credentials removal

See merge request !25

update

See merge request !27

Create extension function to retrieve public keys from Signer service

Closes #30

See merge request !23

update

See merge request !26

Remove explicit user/pass credentials for MongoDB

See merge request !22

init

See merge request !21

Create tasklist extension function

Closes #31

See merge request !20

Use policy data field during execution

Closes #29

See merge request !19

Policy bundles convention in sync app

Closes #28

See merge request !18

Allow passing evaluationID when evaluating a policy

Closes #27

See merge request !17

Store all policy evaluation results in the cache with unique evaluation ID

Closes #25

See merge request !16

Rego built-in function to create task

Closes #24

See merge request !15

Rego extension function for DID resolving

Closes #18

See merge request !14

Make policy parameters order equal in all places

Closes #21

See merge request !13

Sync policy git repository with MongoDB policy collection upon policy changes

Closes #20

See merge request !9

Rego functions for cache write and read (set and get)

Closes #17

See merge request !11

(kalashnikov) Rego cache functions

See merge request !12