Allow custom pageblocks' attributes to be specified via import_from_dict

[nikolas]

To avoid having to do things like this when testing the pageblock:

# Set the check-in block's setting block to the one we just
# created.
goalcheckinblock.goal_setting_block = goalsettingblock
goalcheckinblock.save()
assert(goalcheckinblock.goal_setting_block is not None)