Use jellybeans' diff colours

Jellybeans' diff colours are bright and vibrant and desert's are ugly
and dirty.

I edited the jellybeans code, replaced 'exec' with 'echo' and :redir the
output so I could get the :hi commands.
Tweak the colours to allow syntax highlighting

I want to see syntax in added and changed text, so remove guifg=NONE and
darken the green DiffAdd background.