Migrate ensime to AutoPlugin

Resolves https://github.com/ensime/ensime-sbt/issues/53

This PR migrates ensime to the new [`AutoPlugin`](http://www.scala-sbt.org/release/docs/Plugins.html#Creating+an+auto+plugin) format.

Other changes included in this PR:
- Updated README with new installation instructions and SBT 0.13.5 requirement
- Updated README with real example for `EnsimeKeys.additionalSExp`
- Re-introduce package org.ensime to avoid potential AutoPlugin clashes
- Use conventional Import object for autoImport
- Clean up build.sbt with `publishMavenStyle := false` and use `isSnapshot.value` instead of checking version string.