SPARK-1205: Clean up callSite/origin/generator.

This patch removes the `generator` field and simplifies + documents
the tracking of callsites.

There are two places where we care about call sites, when a job is
run and when an RDD is created. This patch retains both of those
features but does a slight refactoring and renaming to make things
less confusing.

There was another feature of an rdd called the `generator` which was
by default the user class that in which the RDD was created. This is
used exclusively in the JobLogger. It been subsumed by the ability
to name a job group. The job logger can later be refectored to
read the job group directly (will require some work) but for now
this just preserves the default logged value of the user class.
I'm not sure any users ever used the ability to override this.

Author: Patrick Wendell <pwendell@gmail.com>

Closes #106 from pwendell/callsite and squashes the following commits:

fc1d009 [Patrick Wendell] Compile fix
e17fb76 [Patrick Wendell] Review feedback: callSite -> creationSite
62e77ef [Patrick Wendell] Review feedback
576e60b [Patrick Wendell] SPARK-1205: Clean up callSite/origin/generator.