add Mussa::save_motif()

add Mussa::save_motif()
necessary for ticket:133
This adds the low level motif saving code to Mussa. To make things
work a bit more reliably, the parse code can understand the alpha
channel now (so I can save hidden motifs), and can parse a
motif name that is quoted like "foo asdf". I added that as I expect
users will put spaces in their motif names.

Although things'll probably break if someone puts a " in a motif name.
see ticket:149