Simplify and fix pyspark script.

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).