Add util.BuildInformation and buildinfo command

This is a first attempt at #41.

- adam-core: Add git-commit plugin to POM file. The file is generated
is `adam-core/src/main/resources/git.properties` and the info is
available in the object
`edu.berkeley.cs.amplab.adam.core.util.BuildInformation`.
- adam-cli: Add the command `buildinfo`.
Merge pull request #138 from smondet/git_properties

Display Git commit info on command line
Refactored SequenceDictionary aggregation to eliminate replication between RDD extensions, and to improve speed. Additionally, refactored SequenceDictionary class to reduce GC overhead and to make class immutable.