Add developers keySetting

The motivation for this is mainly to avoid having to define a pomExtra to publish to Maven central
Merge branch '0.13' of github.com:sbt/sbt into 0.13
Add constructor to avoid binary compatibility issues
Add missing default option
Merge branch '0.13' of github.com:sbt/sbt into 0.13
Add notes
Change additional constructor to what it should probably be

I am at a total loss for why this is not compiling.
Merge branch '0.13' of github.com:sbt/sbt into 0.13
Fix obtuse compilation error
Merge branch '0.13' of github.com:jedesah/sbt into 0.13

Conflicts:
	ivy/src/main/scala/sbt/IvyInterface.scala
Merge pull request #1590 from jedesah/0.13

Add developer key