HTTP endpoint to retreive task results

Closes #12

See merge request !7

* Create task unit tests
* Retrieve task results unit tests

Task execution of independent tasks

Closes #6 and #8

See merge request !4

We changed the design, so now tasks are either executing
a request policy, or an HTTP request to a predefined URL,
but not both.

In the previous commits "request policy" could only be used as
a request transformation and the actual task was executing an
HTTP request to a predefined URL. With the current change tasks
can execute "request policy" if it's set, or make an HTTP call.

Initial service documentation in README.md

Closes #9

See merge request !6

Queue-like interface for retrieving tasks from storage

Closes #7

See merge request !3

Create task HTTP endpoint

Closes #5

See merge request !2

Service skeleton with health check endpoints

Closes #1

See merge request !1