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.
I don't like this test in the middle.  Move this up (after egyptian_fraction(Rational(3, 7)), maybe add a commentary.  Or to the bottom, after Golomb test.  Or just remove this.