Fix/vik/oe pep8

Add logging to debug strange cohort behavior on prod.

do a bunch of pylint improvements

Feature/jkarni/folditx

Strange behavior:
  - There are 20 cohorts that users should be randomly assigned
    into in HeroesX
  - Almost all, but not all (e.g. ~3300 / 3400) users end up in the
    same group.
  - testing manually locally and on prod in a django shell shows
    nothing wrong

Fix modulestore tests for pep8 violations

Feature/cdodge/pylint cleanup

  Less is more.

Initial implementation of htmlbook.

Make numeric response handle plus sign exponential correctly

delete_item should update the parent's list of children to reflect the d...

NumericResponse was failing to accept as correct an answer with a plus
sign in the exponential. For example: 5.0e+1 was marked as incorrect
if the answer was 50. However, 5.0e1 was marked as correct. This
caused confusion, since the answer shown to students included the plus
sign.

LMS Lighthouse [#242]

Add the ability to hide correctness markers

Fix/cdodge/performance optimize comment client use jumpto

delete_item should update the parent's list of children to reflect the delete operation. Also added unit test

Specify that mitx should use ruby 1.9.3-p374 for consistancy with prod

Update of Delft logo (lighthouse ticket #234)