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.
Update documentation for AutoPlugins.

* Add notes about AutoPlugins vs. RootPlugins in Plugins section
* Modify best practices section to recomend using AutoPlugins.
* Modify Using-Plugins section in getting started to denote
  auto plugins.
Add note about not exposing fine-grained autoplugin inclusion controlls.

AddSettings should only expose coarse-grained features of AutoPlugins
or else the Logic we use to ensure safe addition completely breaks
down.  Leaving it in the code as an escape hatch if we get desparate,
but we need an alternative for controlling ordering later.