Add functions to continued_fraction.py, fix small problems.

Happened to find sympy just when I was in the middle of doing a bunch of coding for continued fractions...

Fixed small issues with continued fractions (only the first term may be negative, and handle quadratic irrationals which are rational after all, because their discriminator is a perfect square).  Also added some new CF functions (names maybe could be better) for generating convergents of a CF from an iterator, or evaluating it as a Rational.

Hope these are useful.