Add developers keySetting

The motivation for this is mainly to avoid having to define a pomExtra to publish to Maven central
Remove some compiler warnings
Merge branch '0.13' of github.com:sbt/sbt into 0.13
Merge branch 'refs/heads/0.13' into topic/remove-warnings
Add constructor to avoid binary compatibility issues
Untested PR for #485. Unable to test because unable to build
Merge remote-tracking branch 'upstream/0.13' into 0.13
Add missing default option
Tested PR for #485. Had a hard time squashing this, maybe because I did a merge in between. Sorry about that!
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.
Expose Ivy's circular dependency check via UpdateOptions
Bump to a version of ivy that will not swap in httpclient URL handler.

When ivy finds the httpclient on the classpath, it will silently change
its url handler, and the httpclient in ivy 2.3.x has a few known
issues.
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 #1601 from sbt/wip/circular

Expose Ivy's circular dependency check via UpdateOptions
Add notes for this fix.
Merge pull request #1602 from sbt/wip/ivy-no-commons-httpclient

Bump to a version of ivy that will not swap in httpclient URL handler.