Migrate ensime to AutoPlugin

- Re-introduce package org.ensime
- Use conventional Import object for autoImport
- Clean up build.sbt with `publishMavenStyle := false` and use isSnapshot
Merge pull request #55 from fommil/master

regression in last patch: inferred name of project
Merge pull request #54 from markschaake/auto-plugin

Migrate ensime to AutoPlugin
revert example custom options