HTTP endpoint to retreive task results

@yordan.kinkov @valerii.kalashnikov

The endpoint allows to retrieve task results only by using a taskID, because the task metadata contains the cache namespace and scope which were used to store the result in cache.

The results can also be retrieved by using the Cache service directly (without using this endpoint), but the client will have to know in advance the cache namespace and scope for the given task.

Closes #12 (closed)