Don't send SIGINT to Py4J gateway subprocess.

This addresses SPARK-885, a usability issue where PySpark's
Java gateway process would be killed if the user hit ctrl-c.

Note that SIGINT still won't cancel the running s

This fix is based on http://stackoverflow.com/questions/5045771
RDD sample() and takeSample() prototypes for PySpark
Change docs color theme for 0.8
New hardware provisioning doc, and updates to menus
More doc improvements + better warnings when you haven't built Spark
Update docs about HDFS versions
Merge pull request #870 from JoshRosen/spark-885

Don't send SIGINT / ctrl-c to Py4J gateway subprocess
Merge pull request #861 from AndreSchumacher/pyspark_sampling_function

Pyspark sampling function
More updates, describing changes to recommended use of environment vars
and new Python stuff
Delete some code that was added back in a merge and print less info in
spark-daemon
Various web UI improvements:

- Use "fluid" layout that can expand to wide browser windows, instead of
  the old one's limit of 1200 px
- Remove unnecessary <hr> elements
- Switch back to Bootstrap's default theme and tweak progress bar colors
- Make headers more consistent between deploy and app UIs
- Replace some inline CSS with stylesheets
Print output from spark-daemon only when it fails to launch
Fixes suggested by Patrick
Another fix suggested by Patrick
Small tweak
Merge pull request #877 from mateiz/docs

Doc improvements for 0.8
Small fixes to README
Merge pull request #880 from mateiz/ui-tweaks

Various UI tweaks
Better docs
Add assmebly plug in links