Remove Natures from AutoPlugins feature.

* remove the notion of Natures from Autoplugins.
* Update tests to use AutoPlugins with no selection for inclusion.
* Rename exisitng Natures code to Plugins/PluginsDebug.
Allow Build.scala project settings to be ordered via AddSettings.

* Create new AddSettings.ProjectSettings that can be used in the Addsettings order.
* Update Load.scala to correctly abide by AddSettings orderings.
AutoPlugins appropriately participate in AddSettings.

* Add new AutoPlugins type to AddSettings.
* Ensure any Plugins filter doesn't just automatically always add
  autoplugins every time.
* Load.scala can now adjust AutoPlugins ordering

Note: Adjusting autoplugin ordering is dangerous BUT doing a glob
      of "put autoplugin settings here" is generally ok.
Split Defaults.scala into three plugins: Global,Ivy,Jvm.

* GlobalPlugin has defaults for controlling parallelism on tasks, basic command stuff.
* IvyModule has the configuration for resolving/publishing modules to ivy, assuming
  each project is a single module.
* JvmModule has the configuration for compiling/running/testing/packaging Java/Scala
  projects.
Remove defaultSettings usage from the docs.

Given the addition/promotion of AutoPlugins, remove
the references to the previously necessary
Defaults.defaultSettings method.