SPARK-1314: 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 incidentally 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 (previously, there was no support for adding Hive to any of our jars produced by Maven)
- assemble-deps fixed since we no longer use a different ASSEMBLY_DIR
- avoid adding log message in compute-classpath.sh to the classpath :)

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.

Author: Aaron Davidson <aaron@databricks.com>

Closes #237 from aarondav/master and squashes the following commits:

5dc4329 [Aaron Davidson] Typo fixes
dd4f298 [Aaron Davidson] Doc update
dd1a365 [Aaron Davidson] Eliminate need for SPARK_HIVE at runtime by d/ling datanucleus from Maven
a9269b5 [Aaron Davidson] [WIP] Use SPARK_HIVE to determine if we include Hive in packaging