Make Egyptian fractions a little more robust.

Add checking for negative arguments, and arguments > 1

Don't know if this is actually needed; it does help avoid endless loops if you give an improper fraction to some of the algorithms.  Feel free to reject.
> Are the ones I put in test_ntheory.py not enough? 

Which one test this line?

> You didn't like the doctest. 

Doctests are not tests, just examples for reader.