Fix broken annotation processor default discovery

- Move `javax.annotation.processing.Processor` file to `services` folder since this is the location where it is supposed to be per [docs](http://docs.oracle.com/javase/1.5.0/docs/guide/apt/GettingStarted.html), before it was in the `service` folder. This was breaking the default discovery procedure for apt.
- Reflect changes in processor POM. I must say I've copied & pasted the maven compiler plugin config from dagger, it looks like it's working just fine for Memento as well but you might want to have a closer look at it.