(PE-2843) Make it possible to get a reference to a service from a service

Sometimes instead of just getting a reference to the functions from a
dependency service in TK, it is more useful to get a reference to the
service itself (e.g. the object that `satisfies?` the service protocol).
This commit adds a `get-service` function to the main `Service` protocol in TK.
Throw an error from get-service if service does not exist
Remove commented line
Merge pull request #117 from cprice404/feature/master/PE-2843-service-get-service

(PE-2843) Make it possible to get a reference to a service from a service