Make RGB(::Integer,::Integer,::Integer) make sense.

Is there anything I'm missing, or does 

```
RGB(r::Integer, g::Integer, b::Integer) = RGB{Float64}(r, g, b)
```

not make any sense because, according to the comments in `RGB`, the `r`, `g` and `b` values are within [0,1] and thus this line only permits to construct fully saturated colors?

This PR at least makes `RGB(0xa4,0x39,0x1f)` do what one'd expect, though the result of `RGB(164,57,31)` would still be unexpected. We could just always divide by `255` to make the latter one work, but that'd make handling 48 or 96 bit colors wrong. I'm in favor of 255 for the "least surprise."