[imp #20] Implement change stream as Observable

Using rxjava-scala, we are able to provide a high-level, nice-to-use API
for being notified about changes in the database. By combining the power
of akka actors and `Observables` such streams can be combined and worked
with in any fashion the user wants it.

closes #20
Merge pull request #21 from gnieh/wip-observable

[imp #20] Implement change stream as Observable