Remove dependence on command-line tools

[za3k]

Remove dependencies on all command-line tools:

• gpg
• zbarimg (+convert, +grep) (See #10 #9 also)
• zbarcam
• (which)

[anarcat]

(which)

i've looked at this in the code and i think the proper way to do this is simply not check if the program is present and try to execute it directly. you will get an error and then you handle that appropriately.

i was about to go on and do this, but there's calls like this everywhere in the code and I wanted to make sure that made sense for you before going ahead. :)

[za3k]

I'm wasn't planning to remove dependence on which. The reason being, if I just remove dependency on all the other command-line tools, the which dependency disappears automatically.

That said if you give me a PR I'd accept it.

Edit: I still want to fail-fast for the new version, so make sure to preserve that.