Unifies AutoPlugin and AutoImport. Fixes #1188 (resend)

I'm resending #1198 since target branch needs to be changed to 0.13.

See #1188 for details. This pull req implements @jroper's `def autoImport: Any` idea. 

* `trait AutoImport` is subsumed by `def autoImport` method under `AutoPlugin`.
* When `def autoImport` is overridden by a `lazy val` or a `val`, `*.sbt` automatically imports `autoImport._`.