Give a group-based name to magic-key buffers

* magit-key-mode.el: Give a specific name to magic-key buffers according to group used.
(magit-key-mode-buf-name): Now a format string.
(magit-key-mode-last-buffer): New internal var.
(magit-key-mode): Create a new buffer name specifying the group in use.
(magit-key-mode-kill-buffer): Kill magit-key-mode-last-buffer.
Remove a defvar declaration

* magit-key-mode.el (magit-log-mode-window-conf): Remove one defvar declaration which is already defined at top level.
introduce commit filed for GPG signature

bind to C-c C-S by default.

Note that using "git -S" from Emacs might require writing a gpg wrapper
and setting git config gpg.program, as there is no control over the gpg
options that are used.
refactor magit-wash-log-line

split the parsing logic for easier extensibility
add ability to display gpg status
Unifies the naming of magit buffers

Adds a space between 'magit-key:' and the name of the command.
This is more consistent with the way the status buffers are named.