[WIP] Use SPARK_HIVE to determine if we include Hive in packaging

Previously, we based our decision regarding including datanucleus jars
based on the existence of a spark-hive-assembly jar, which was incididentally
built whenever "sbt assembly" is run. This means that a typical and
previously supported pathway would start using hive jars.

This patch has the following features/bug fixes:

- Use of SPARK_HIVE (default false) to determine if we should include Hive
  in the assembly jar.
- Analagous feature in Maven with -Phive.
- assemble-deps fixed since we no longer use a different ASSEMBLY_DIR

Still TODO before mergeable:
- We need to download the datanucleus jars outside of sbt. Perhaps we can have
  spark-class download them if SPARK_HIVE is set similar to how sbt downloads
  itself.
- Spark SQL documentation updates.