Final Pynac Switchover Push for Sage-4.0

_pow_ in number_field_element and _rational_ has some hacks since the Pynac library will cause infinite recursions.
Handling of fderivatives so that we can convert to Maxima.
There's a lot of "grunt work" to be done to fix doctests to work with the new output format.

In each case somebody has to verify that the two things are equal.

The massive speed regressions, making pynac symbolics *slower* than Maxima for some benchmarks.
The current handling of I as a wrapper around a number field element is a bit awkward since (I+2)*x can't be expanded as I+2 is "atomic". This also means that I.imag() returns 1.0 instead of 1. This causes issues in algebraic.