Allow to use different actor system

I'd like to propose some changes to scamandrill:
1. Allow to use own  actorSystem 
2. Chanege clients from objects to classes - to allow point 1.
3. Remove unnecessary dependences from compile scope, drop unnecesary deps
4. Upgrade dependencies versions
All of that makes it easier for me to use scamandrill as a good citizen of my bigger akka application