Update doctesting framework

[robertwb]

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.

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