add a `swatch.dumps` method which builds a well-formed ASE header

it does not do any checking of the input object, it just writes the main header that indicates how many blocks you're about to consume'
writer can produce Gray swatches

it doesn't support any color modes other than gray
and it doesn't yet support color groups
parse other types of swatch objects (rgb, cmyk, lab)

the only thing holding this back was the ability to correctly pad
the color mode, it otherwise works just the same as with the gray
swatches