This commit makes Spark invocation saner by using an assembly JAR to
find all of Spark's dependencies instead of adding all the JARs in
lib_managed. It also packages the examples into an assembly and uses
that as SPARK_EXAMPLES_JAR. Finally, it replaces the old "run" script
with two better-named scripts: "run-examples" for examples, and
"spark-class" for Spark internal classes (e.g. REPL, master, etc). This
is also designed to minimize the confusion people have in trying to use
"run" to run their own classes; it's not meant to do that, but now at
least if they look at it, they can modify run-examples to do a decent
job for them.

As part of this, Bagel's examples are also now properly moved to the
examples package instead of bagel.

- Exclude a version of ASM 3.x that comes from HBase
- Don't use a special ASF repo for HBase
- Update SLF4J version
- Add sbt-dependency-graph plugin so we can easily find dependency trees

Add zeroLengthPartitions() test to make sure, e.g., StatCounter.scala can handle empty partitions without incorrectly returning NaN

- akka  2.0.3  → 2.1.0
- spray 1.0-M1 → 1.1-M7
For now the repl subproject is commented out, as scala reflection api changed very much since the introduction of macros.

By default - I'm leaving this commented out. This is because
there is a bug in the PGP signing plugin which causes it to active
even duing a publish-local. So we'll just uncomment when we decide
to publish.

This does a few things to get this branch ready for release:

1. Upgrades the sbt and Scala version
2. Sets the release number to 0.5.1
3. Adds the Sonatype publishing target
4. Installs the PGP signing plugin
5. Removes the Mesos jar dependency