Minor build enhancements

A few changes related to the build including

1) lock down sbt version to 0.12.0. I tend to use the latest version of sbt where ever I go but not everyone project I load up uses the latest version of sbt. This is not a bad thing but when a project is configured with plugins, sbt depends on them as binary dependencies. You'll get a warning if you're using something like 0.12.3. With sbt 0.13.0 around the corner, if someone boots up this project using that they are likely to get stuck when the project loads up the 0.12 plugins. This can create a bad first experience so, in general, if a project uses plugins, its always best to lock down the the projects sbt version to the version the plugins were build for. In this case 0.12.0 in project/build.properties

2) prefer constants over copy/pasta. I'm not sure why they aren't more wildly used, but sbt provides constants for common setting values that normally get cut and pasted from various projects. I updated the build to use those constants.