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.