Remove activation of profiles by default

See the discussion at https://github.com/mesos/spark/pull/355 for why
default profile activation is a problem.
SPARK-673: Capture and re-throw Python exceptions

This patch alters the Python <-> executor protocol to pass on
exception data when they occur in user Python code.
Merge pull request #431 from mbautin/revert_default_profile

Remove activation of profiles by default
Some style cleanup
Small fix from last commit
Merge pull request #434 from pwendell/python-exceptions

SPARK-673: Capture and re-throw Python exceptions
Merge branch 'mesos'