Update doctesting framework

[Robert Bradshaw]

There are several improvements that would be good to make, including but not limited to:

1. Forking rather than starting up a new Sage each time.

2. Customizeable comparison rather than mutating doctests into new doctests (e.g. followup to #10952).

3. Eliminate need to write temporary files for every doctested file.

4. Provide mechanisms for recording and comparing timings.

5. Constant headaches caused by the difference in doctesting library vs. non-library files.

6. The ability to drop into a debugger if a doctest raises an exception.

Code at

​http://code.google.com/p/sagemath-timer/

does most of the above, should be added to the sage library.

[David Roe]

I'm working on this. E-mail me if you want to collaborate.