devel/emacs: add devel/try-emacs-mua

devel/try-emacs-mua provides an easy way to try and experiment
with the notmuch emacs client distributed in emacs subdirectory of
the notmuch source tree.

try-emacs-mua starts a new emacs process and if initial checks pass
*scratch* buffer is filled with information of how to begin.

Normal emacs command line arguments can be used, like -q or -Q.
These arguments are appended verbatim to the starting emacs process.

If the emacs version in use is smaller than 24.4, special care is taken
to ensure that notmuch*.elc files older than corresponding .el files
are not loaded. Since emacs 24.4, setting `load-prefer-newer' variable
takes care of this.