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.
No, but this is OUR interface, so this is fine.  We may hide ivy even further in the future, in which case this location becomes "canon" for dependnecy management in sbt, so yes this is right.