Maven Central Publishing config

I haven't actually verified that this works since of course I don't have your Sonatype credentials, but I thought I'd help get the ball rolling. I emulated [the metadata from KeenClient-Java][1] so I believe things are mostly correct here. A few things to note though:

- License is set as Apache 2 because that's what KeenClient-Java is using, but #22 is for MIT, so need to settle that matter.
- It'd be nice if we could reset the version number to sub-1.0 before beginning to publish artifacts—the `README` warns that the API is still in flux, and I think we shouldn't commit to SemVer backwards compatibility promises yet (#20 and some follow-up I have in the works being a few reasons :smile:). Since users haven't been able to install published artifacts before I'm guessing this wouldn't upset anyone too much—switching from an unmanaged dependency to adding a new managed one to build config will be a conscious decision where people ought to be reasonably expected to check changelogs.

For reference, to test/use this, make sure you set Keen's Sonatype credentials in `~/.ivy2/.credentials` [as described here][2] and that you have a published PGP key for signing [as described here][3], then in theory `sbt publishSigned` should work. It's okay to run a test as it's only the first staging step of [the two-part public release process][4].

[1]: https://github.com/keenlabs/KeenClient-Java/blob/master/gradle.properties
[2]: http://www.scala-sbt.org/0.13/docs/Publishing.html#Credentials
[3]: http://www.scala-sbt.org/0.13/docs/Using-Sonatype.html
[4]: http://central.sonatype.org/pages/releasing-the-deployment.html