To begin: Make a redstone clock of your choice, With either 2 or 3 command blocks coming off it.
You probably knew this was possible, but It's fairly hard.
Step 1: Get the co-ords of your sign, and which way it's facing eg. east .etc
Paste the commands into the different blocks. NOTE: The /setblock is only for anti-grief, you don't have to have it.
NOTE 1: The ~ ~ ~ means co-ords of your sign
NOTE 2: Write your text between the ""s
NOTE 3: The [1-15] is the direction the sign will be facing. I don't know the entire thing, so you'll have to trial and error
My hopper clock, Result 1 and Result 2 (Not in order)http://postimg.org/image/hx2z9u0tl/