Make recent submissions more likely to be picked

Made a few different changes to make a first pass at this - happy to change any/all:

* Make the likelihood of a submission being randomly chosen for
  nitpicking deteriorate over time.
* Give a 30% chance that an exercise other than the last one you
  completed will be chosen as one for you to nitpick.
* No longer take number of nits into account when selecting an exercise.


The algorithm used is the second half of a normal distribution shifted such that it's center is at 0 as it seemed to give a reasonably friendly drop-off curve. This is what it looks like (the most recent submissions are at the top of the list, and although it tails off before the end the last submissions do sometimes still get chosen - it just doesn't show here).

![tinygrab screen shot 13-11-2013 17 49 39](https://f.cloud.github.com/assets/1231200/1533863/2cb7b9f4-4c8c-11e3-8a8d-2b62ec954323.png)

I'm a bit nervous about testing this as I've been able to test the algorithm in isolation as well as that it hasn't obviously broken anything when in the site, but I guess the only way we'll know if it is *behaving* as we want is when it's actually in production... :worried:

While I was at it I made it such that exercises other than the last one you completed aren't excluded, either.
There were five tests to do with markdown I couldn't get passing - but I don't have them passing in master either.

This will close #965