add func & test for getting numerator & denominator of fixed point decimal number

[michiboo]

Address issues #56

[coveralls]

Coverage increased (+0.02%) to 97.531% when pulling b72a32b on michiboo:den_num into e8c9f3b on JuliaMath:master.

[NHDaly]

This seems nice to me. Thanks for the PR! Many apologies for our very long delay.

It's probably worth considering whether there could be a more performant way to do this..

For example, we could start by constructing the rational via Rational(x.i, coefficient(FD{T, f})), and then take its num and dec. I think that would probably be more performant, since it avoids unnecessarily converting to a Float in between?

    return Rational(x.i, coefficient(FD{T, f})).num

Something like that, i think, anyway.

[NHDaly]

That said, i'm happy to merge like this if we want to, as well