Issue54

Fixes #54 .

This PR is based on #59, so #59 should probably be merged first.

Adds to the Fraction class:
- Methods for division, negation, inversion, exponentiation, sign, abs, geometric abs, floor/ceiling/round
- Constructors which take more kinds of numbers
- {int|long|float|double}Value() methods, which throw an exception if the Fraction can't be represented appropriately in teh requested type.