Add new SBT target for dependency assembly
Add a comment and exclude tools
Merge branch 'master' of https://github.com/apache/incubator-spark into sbt-assembly-deps
Exclude assembly jar from classpath if using deps
Rename SBT target to assemble-deps.
Spark shell exits if it cannot create SparkContext

Mainly, this occurs if you provide a messed up MASTER url (one that doesn't match one
of our regexes). Previously, we would default to Mesos, fail, and then start the shell
anyway, except that any Spark command would fail.
Merge pull request #71 from aarondav/scdefaults

Spark shell exits if it cannot create SparkContext

Mainly, this occurs if you provide a messed up MASTER url (one that doesn't match one
of our regexes). Previously, we would default to Mesos, fail, and then start the shell
anyway, except that any Spark command would fail. Simply exiting seems clearer.
Merge pull request #68 from mosharaf/master

Faster and stable/reliable broadcast

HttpBroadcast is noticeably slow, but the alternatives (TreeBroadcast or BitTorrentBroadcast) are notoriously unreliable. The main problem with them is they try to manage the memory for the pieces of a broadcast themselves. Right now, the BroadcastManager does not know which machines the tasks reading from a broadcast variable is running and when they have finished. Consequently, we try to guess and often guess wrong, which blows up the memory usage and kills/hangs jobs.

This very simple implementation solves the problem by not trying to manage the intermediate pieces; instead, it offloads that duty to the BlockManager which is quite good at juggling blocks. Otherwise, it is very similar to the BitTorrentBroadcast implementation (without fancy optimizations). And it runs much faster than HttpBroadcast we have right now.

I've been using this for another project for last couple of weeks, and just today did some benchmarking against the Http one. The following shows the improvements for increasing broadcast size for cold runs. Each line represent the number of receivers.
![fix-bc-first](https://f.cloud.github.com/assets/232966/1349342/ffa149e4-36e7-11e3-9fa6-c74555829356.png)

After the first broadcast is over, i.e., after JVM is wormed up and for HttpBroadcast the server is already running (I think), the following are the improvements for warm runs.
![fix-bc-succ](https://f.cloud.github.com/assets/232966/1349352/5a948bae-36e8-11e3-98ce-34f19ebd33e0.jpg)
The curves are not as nice as the cold runs, but the improvements are obvious, specially for larger broadcasts and more receivers.

Depending on how it goes, we should deprecate and/or remove old TreeBroadcast and BitTorrentBroadcast implementations, and hopefully, SPARK-889 will not be necessary any more.
Merge pull request #66 from shivaram/sbt-assembly-deps

Add SBT target to assemble dependencies

This pull request is an attempt to address the long assembly build times during development. Instead of rebuilding the assembly jar for every Spark change, this pull request adds a new SBT target `spark` that packages all the Spark modules and builds an assembly of the dependencies.

So the work flow that should work now would be something like

```
./sbt/sbt spark # Doing this once should suffice
## Make changes
./sbt/sbt compile
./sbt/sbt test or ./spark-shell
```