Optionally parallelize the Spark build.

This introduces a new environment variable, `SPARK_BUILD_THREADS`,
that controls the level of Maven parallelization (if set). This uses
Maven 3's syntax for the number of threads: either a simple integer or
something like `1.5C` to indicate 1.5 times the number of cores on the
build server.

On my hardware, (Intel Xeon E5-1620 v2, roughly equivalent to a fast
i7), a setting of `1.5C` speeds up the build by about 15%.

This will trigger some warnings that the Scala plugins are not marked as
threadsafe. I suspect that they *are* threadsafe, but I don't have any
references or extensive testing to prove this.