Plain-text report of per-question scores

[papajohn]

Right now, the only way to render the result of scoring is as HTML via gofer.ok.OKTestsResult._repr_html_. A __repr__ method that generates an equivalent plain-text version that's nicely formatted would be awesome. Right now, if you run grade_notebook from the command line, the description isn't useful. E.g.,

Question 1:
<gofer.ok.OKTestsResult object at 0x107b3fe48>
Question 2:
<gofer.ok.OKTestsResult object at 0x107b3f390>