Fix show for Ufixed

This fixes a bug that cropped up in the display of methods written in terms of `T<:Ufixed`. Since you can't call `nbitsfrac` on a `Ufixed` (it needs to be a `UfixedBase`), it triggered an error.

Well, we can get rid of it.

The main motivation was that, without it, looking at a 5x5 block of an image gives you this:
```
5x5 Array{RGB{UfixedBase{Uint8,8}},2}:
 RGB{UfixedBase{Uint8,8}}(Ufixed8(0.467),Ufixed8(0.212),Ufixed8(0.871))  RGB{UfixedBase{Uint8,8}}(Ufixed8(0.353),Ufixed8(0.6),Ufixed8(0.804))    …  RGB{UfixedBase{Uint8,8}}(Ufixed8(0.431),Ufixed8(0.796),Ufixed8(0.522))
 RGB{UfixedBase{Uint8,8}}(Ufixed8(0.075),Ufixed8(0.392),Ufixed8(0.984))  RGB{UfixedBase{Uint8,8}}(Ufixed8(0.231),Ufixed8(0.31),Ufixed8(0.412))      RGB{UfixedBase{Uint8,8}}(Ufixed8(0.337),Ufixed8(0.349),Ufixed8(0.361))
 RGB{UfixedBase{Uint8,8}}(Ufixed8(0.851),Ufixed8(0.62),Ufixed8(0.043))   RGB{UfixedBase{Uint8,8}}(Ufixed8(0.62),Ufixed8(0.333),Ufixed8(0.6))        RGB{UfixedBase{Uint8,8}}(Ufixed8(0.102),Ufixed8(0.737),Ufixed8(0.212))
 RGB{UfixedBase{Uint8,8}}(Ufixed8(0.671),Ufixed8(0.91),Ufixed8(0.529))   RGB{UfixedBase{Uint8,8}}(Ufixed8(0.596),Ufixed8(0.251),Ufixed8(0.918))     RGB{UfixedBase{Uint8,8}}(Ufixed8(0.125),Ufixed8(0.424),Ufixed8(0.067))
 RGB{UfixedBase{Uint8,8}}(Ufixed8(0.365),Ufixed8(0.867),Ufixed8(0.729))  RGB{UfixedBase{Uint8,8}}(Ufixed8(0.22),Ufixed8(0.49),Ufixed8(0.616))       RGB{UfixedBase{Uint8,8}}(Ufixed8(0.584),Ufixed8(0.8),Ufixed8(0.686))
```
which is quite a mouthful. But I suppose the better approach is to change this in terms of how ColorValues (not types) are represented?