Add aggregation and display of metrics obtained from Spark

This change adds an new command-line option called "print_metrics" which causes a listener to be registered with Spark, which accumulates metrics when each task and stage end. When a command is complete the metrics are logged to stdout.

Some notes:

- The "print_metrics" option is added in the Args4jBase class. I thought about adding it in SparkArgs, but the intention is that it can be used for non Spark-related metrics in the future.
- The metrics are currently logged to stdout. They could be logged to SLF4J instead if that's better -- let me know and I'll change it if necessary.
- I've added a dependency on this library: http://code.google.com/p/java-ascii-table/ for printing ascii tables (Apache-licensed). It's not on any public maven repos, so I've added a local maven repo at /repo which contains the JAR file and POM. This seems to work fine both with the command line and Intellij, but let me know if there are any issues.
Small nit, but it might be preferable to disambiguate the name Header here as there is code in a few other places in ADAM that deals with other headers (e.g., SAM/VCF headers).