Allow_repeats prevent same condition for a given worker

[ahafri]

Add option to prevent repeating same condition

Implement functionality that does the following:

• Extend the "allow_repeats" functionality to prevent participants from repeating the same
  condition with a new config option, "allow_repeat_conditions" (defaults to false)
• Track which conditions the participant has already participated in, so that once they
  have done all conditions, psiturk prevents them from repeating the experiment again
• Add error message if they have completed all conditions
• Make the number of conditions already done, and total number of conditions, accessible
  to the experimenter in script, so they can display this to the participant if they so choose

Additional functionality implemented (that goes along with allow_repeats functionality):

• if they repeat the HIT, a new config option "always_show_instructions" controls whether
  instructions are skipped over if the participant is repeating the HIT

This is especially useful for large rating studies, where one might want to keep the
minimum number of items rated per person short (to avoid fatigue), but give enthusiastic
participants a chance to continue rating more items. (Here, the experimenter would need to
make the condition correspond to the set of items to rate).

Note: Documentation has not yet been added.

Adds functionality for: #321
See also: #90, #214