This patch removes compatibility for IPython < 1.0 but fixes the launch
script and makes it much simpler.

I tested this using the three commands in the PySpark documentation page:

1. IPYTHON=1 ./pyspark
2. IPYTHON_OPTS="notebook" ./pyspark
3. IPYTHON_OPTS="notebook --pylab inline" ./pyspark

There are two changes:
- We rely on PYTHONSTARTUP env var to start PySpark
- Removed the quotes around $IPYTHON_OPTS... having quotes
  gloms them together as a single argument passed to `exec` which
  seemed to cause ipython to fail (it instead expects them as
  multiple arguments).

Also changed uses of "job" terminology to "application" when they
referred to an entire Spark program, to avoid confusion.

and new Python stuff

are now needed

Added checkpointing and fault-tolerance semantics to the programming guide. Fixed default checkpoint interval to being a multiple of slide duration. Fixed visibility of some classes and objects to clean up docs.

Also, adds a line in doc explaining how to use.

It's nicer if all the commands you need are made explicit.

Expand the PySpark programming guide.