updated steg code

please use the new version only (redownload it)
i did some important things to the steg code which maintains image clarity. (and also speeds up processing ~ 10x)
the old one messed up the images visibly for medium-sized messages.

that is because most coefficients in a jpeg file are empty most of the time.
when we update a 0 to a 1 it is a huge difference in coefficient value. which showed...

what i did:
used
0000 0000 0000 1110 ( == 14)
as the lowest positive value to modify,

and
1111 1111 1111 0001 ( == -15)
as the highest negative value to modify.

(got that idea from PQSTEGO - i think they use different values. mine makes more sense to me)

the effect is that a marginal difference in very few coefficients is untraceable as it should be, rather than a huge relative difference showing up as weird little blocks.

also fixed (COMPLETED...) the burn and test functions.
burn randomises the least significant bit in all useable coefficients (like explained above),
which removes the message completely.

to do still, is work on memory-wiping and stuff like that on exit (so the password / plaintext is not residing in some RAM location somewhere).