Fix typos in AutoPlugin API docs.
minor API updates
Generate error when multiple AutoPlugins provide the same Nature.
move Nature-related classes to Natures.scala
TODO
Restructure Natures/AutoPlugin types

- remove AutoPlugin.provides
 * name comes from module name
 * AutoPlugin is Nature-like via Basic
- Project.addNatures only accepts varags of Nature values
 * enforces that a user cannot explicitly enable an AutoPlugin
 * drops need for && and - combinators
- Project.excludeNatures accepts varags of AutoPlugin values
 * enforces that only AutoPlugins can be excluded
 * drops need for && and - combinators