Add test for BuildPyramid, other minor changes

1. Add unit test for BuildPyramid
2. Build.scala - take Hadoop and Spark versions from environment instead of hardcoding it in Build.scala. So to build Spark on 0.20.2-cdh3u6, simply 
export SPARK_HADOOP_VERSION=0.20.2-cdh3u6
export SPARK_VERSION=0.9.1
./sbt 
...
There are appropriate defaults (look for DEFAULT_HADOOP_VERSION and DEFAULT_SPARK_VERSION)