and made DAGScheduler automatically set SparkEnv.

Added immutable map registration in kryo serializer

Made improvements to takeSample. Also changed SparkLocalKMeans to SparkKMeans

I've fixed the bugs detailed in the diff. One of the bugs was already
fixed on the local file (forgot to commit).

Fixes #105.

(you can no longer iterate over a Source multiple times).

Math.min takes 2 args, not 1. This was not committed earlier for some
reason

merge results into rather than requiring a new object allocation
for each element merged. Fixes #95.

takeSamples method takes a specified number of samples from the RDD and
outputs it in an array.

LocalKMeans runs locally with a randomly generated dataset.
SparkLocalKMeans takes an input file and runs KMeans on it.

The first time they appear, exceptions are printed in full, including
a stack trace. After that, they are printed in abbreviated form. They
are periodically reprinted in full; the reprint interval defaults to 5
seconds and is configurable using the property
spark.logging.exceptionPrintInterval.

When a task throws an exception, the Spark executor previously just
logged it to a local file on the slave and exited. This commit causes
Spark to also report the exception back to the driver using a Mesos
status update, so the user doesn't have to look through a log file on
the slave.

Here's what the reporting currently looks like:

    # ./run spark.examples.ExceptionHandlingTest master@203.0.113.1:5050
    [...]
    11/10/26 21:04:13 INFO spark.SimpleJob: Lost TID 1 (task 0:1)
    11/10/26 21:04:13 INFO spark.SimpleJob: Loss was due to java.lang.Exception: Testing exception handling
    [...]
    11/10/26 21:04:16 INFO spark.SparkContext: Job finished in 5.988547328 s

Before, the cleaner attempted to clone $outer objects that were classes
(as opposed to nested closures) and preserve only their used fields,
which was bad because it would miss fields that are accessed indirectly
by methods, and in general it would confuse user code. Now we keep a
reference to those objects without cloning them. This is not perfect
because the user still needs to be careful of what they'll carry along
into closures, but it works better in some cases that seemed confusing
before. We need to improve the documentation on what variables get
passed along with a closure and possibly add some debugging tools for it
as well.

Fixes #71 -- that code now works in the REPL.