Fix cdpath commit, which was broken in several ways

This fixes commit 04681ff7ceb8a as follows:

 - check when cdpath is None. Otherwise split fails on None objects and
   causes ranger to crashes when CDPATH is not set in the environment.

 - actually set cdpath variable in the lowercase case. Before it was not
   assigned to any variable.

 - join paths with os.path.join instead of concatenating with '/'.
added MATE edition of image and PDF viewer
fix crash in sha encode of previews

When opening certain filetypes, for which Ranger can't render a preview
(they appear as 0 bytes), Ranger will crash on the sha1_encode:

    Traceback (most recent call last):
    File "~/ranger/ranger/core/main.py", line 139, in main
        cacheimg = os.path.join(ranger.CACHEDIR, self.sha1_encode(path))
    File "~/ranger/ranger/core/actions.py", line 821, in sha1_encode
        sha1(path.encode('utf-8')).hexdigest()) + '.jpg'
    AttributeError: 'NoneType' object has no attribute 'encode'

This solves that by checking at the beginning of get_preview() that
`file.realpath` is not None, and returning early if it is None.
rename README to README.md

This way, github treats the README as a markdown file and displays it
nicely.
optimized README.md for github
config/rc.conf: don't use the obsolete -d flag