Publish to sonatype

Prepare scalastic for publishing in Sonatype repo:

* `groupId` is `org.scalastic`, `artifactId` is `scalastic`
* `crossScalaVersions` setting was removed, because grizzled-slf4 1.0.1 version is built only for Scala 2.10. 
`2.10.0` and `2.10.1` versions were redundant. Scala 2.10.0 and later maintains binary compatibility so both 2.10.0 and 2.10.1 will be published as [scalastic_2.10](http://www.scala-sbt.org/0.12.3/docs/Detailed-Topics/Cross-Build.html#publishing-conventions)
* `build.sbt` was converted to `project/Build.scala` in order to have clean settings grouping