Add developer key

The main motivation for this change is to get rid of the mandatory pomExtra that everyone is forced to use to publish to maven central.

As an added benefit, it sould be easy enough to implement a plugin that depends on the git plugin to allow to fill in the developers list using the git commit log.

Originated from this stackoverflow question: http://stackoverflow.com/questions/25630281/is-there-a-reason-why-sbt-does-not-support-the-developer-key
Figured this should be easy enough and I could do it myself instead of just doing a ticket.
Here is where we need to provide a new `def this(... everythign but devleopers)` method to retain binary compatibility, otherwise this can't hit until sbt 1.0