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.
It feels more like a value thing to me.  Looking at http://www.ics.uci.edu/~eppstein/numth/egypt/intro.html , it doesn't always make sense to talk about Egyptian fractions > 1.  Using the divergent sequence is called the Harmonic method there, and changes things greatly (computing 100+1/4 would involve gigantic denominators, just to get up that high).  The Greedy method in his terminology just clips off the whole part and adds it back in later without converting it (so the result wouldn't be an ascending sequence of reciprocals, like we want).  For the Takenouchi method (called the Pairing method in the page), it says "Note that this process may combine pairs of fractions to form integers; e.g. this happens with sufficiently many copies of 1/7. If this happens, we allow integers to be combined to make larger integers." Again, that leads to something unrepresentable in our chosen format (list of ascending integers, reciprocals of the unit fractions).  The "Binary" method also allows whole numbers in the representation... it just isn't consistent.  Either we should allow each individual algorithm to handle large inputs in its own way (or return a ValueError) or we should ValueError for all of them: non-fractional Egyptian fractions just aren't how we do Egyptian fractions, at least if we're returning them in the form we do.

(Now, if we decide to return a descending list of unit fractions, instead of their reciprocals, that might change things.)