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.
Maybe.  But then, according to the definition, you really can't have an Egyptian fraction greater than 1.  The Greedy algorithm (prior to my change) already gives answers like this ([1, 1, 1, 2] or something for 7/2), so I thought I would generalize it to work with the other algorithms also, instead of them going into an infinite loop.  Is there a better way to represent these "illegal" Egyptian fractions?  Would you prefer [1/3, 2]?  Or maybe it would be better just to reject such inputs altogether.