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.
You don't want the default parameters in this version.  That's why you're getting compilation failures, it's a  pretty obtuse error message though:

```
[error] /home/travis/build/sbt/sbt/ivy/src/main/scala/sbt/IvyInterface.scala:14: double definition:
[error] method <init>$default$8:=> Option[sbt.ScmInfo] and
[error] method <init>$default$8:=> Option[sbt.ScmInfo] at line 13
[error] have same type
[error]   def this(nameFormal: String, description: String = "", homepage: Option[URL] = None, startYear: Option[Int] = None, licenses: Seq[(String, URL)] = Nil, organizationName: String = "", organizationHomepage: Option[URL] = None, scmInfo: Option[ScmInfo] = None) =
[error]                                                                                                                                                                                                                                    ^
[error] /home/travis/build/sbt/sbt/ivy/src/main/scala/sbt/IvyInterface.scala:14: double definition:
[error] method <init>$default$7:=> Option[java.net.URL] and
[error] method <init>$default$7:=> Option[java.net.URL] at line 13
[error] have same type
[error]   def this(nameFormal: String, description: String = "", homepage: Option[URL] = None, startYear: Option[Int] = None, licenses: Seq[(String, URL)] = Nil, organizationName: String = "", organizationHomepage: Option[URL] = None, scmInfo: Option[ScmInfo] = None) =
[error]                                                                                                                                                                                          ^
```


Basically, this method is *only* for binary compatibility, so you don't need the syntactic convenience of the default parameters anyway.  We're just trying to make sure binary calls to the old constructor don't explode at runtime :)