Moving over the adam.avdl file from adam-format.

In order to get the bdg-formats repository going, I've moved over the adam.avdl file
wholesale from the adam-format module in the main adam repo.

To avoid conflicts, I've given this repo its own pom.xml with an artifactId of 'bdg-formats',
but carrying over the same version, parent pom, etc.
Merge pull request #1 from genomebridge/tdanford/adam-format-port

Moving over the adam.avdl file from adam-format.