Rename ApplicationBuilder.createChild to addChild

- Adds ApplicationBuilder.addChild(EntitySpec) and 
  ApplicationBuilder.addChild(Map, Class)
- Deprecates the old createChild methods
- Updates all uses
Add guards to ApplicationBuilder for misuse

- throw if doBuild() calls manage()
- throw if concurrent calls to manage() attempted
- throw if tries to modify the appSpec after the app has been
  created (e.g. calling configure(Map))
- improves javadoc
Merge pull request #621 from aledsage/fix/api-ApplicationBuilder-addChild

Rename ApplicationBuilder.createChild to addChild