Integer exponentiation

[danilopedraza]

I implemented the exponentiation on integers badly. When the exponent is non-negative, The result is an integer and I can keep going. When the exponent is negative, I can return a fraction.

[danilopedraza]

The library implements exponentiation between BigInt and BigUInt, something that I haven't noticed.

[danilopedraza]

Done. Integer exponentiation is now implemented for arbitrary integers.

[danilopedraza]

Important note: I don't return a fraction when the exponent is an integer: I return a decimal.