write the algorithms to write to and read from the bitmap

* now we can write to the LSBs of the bitmap using:
        $ ./stegmp write file.bmp input_file

if input_file is "-", then then standard input will be handled

* the same way, we can read the content of the-said LSBs using:
        $ ./stegmp read file.bmp

Signed-off-by: Olivier Gayot <duskcoder@gmail.com>