Avoid calls to stat() in find_matches

Instead of stating every directory to check if its realpath is the
current directory, only do so for any potential matches.
Do not decode os.path.realpath / path

`path` is decoded already (coming from `db`) and this caused the
following error:

    Traceback (most recent call last):
      File "/home/user/.autojump/bin/autojump", line 460, in <module>
        if not shell_utility(): sys.exit(1)
      File "/home/user/.autojump/bin/autojump", line 429, in shell_utility
        results = find_matches(db, patterns, max_matches, False)
      File "/home/user/.autojump/bin/autojump", line 374, in find_matches
        if current_dir == decode(os.path.realpath(path)) :
      File "/home/user/.autojump/bin/autojump", line 277, in decode
        return text.decode(encoding, errors)
      File "/usr/lib/python2.7/encodings/utf_8.py", line 16, in decode
        return codecs.utf_8_decode(input, errors, True)
    UnicodeEncodeError: 'ascii' codec can't encode character u'\xb4' in
    position 52: ordinal not in range(128)
Skip current_dir in find_matches for fuzzy=False, too
Use named args when calling find_matches

This should make it clearer at first glance where True and False is
referring to.
Avoid file stats in find_matches for fuzzy=True, too
Reduce lag for using when using sshfs mounts caused by checking pwd.
Merge pull request #170 from blueyed/gh-find_matches-named-args

Use named args when calling find_matches
merging blueyed's changes to reduce stat() calls
Removed --zsh from README.md, since it seems that install.sh doesn't support it.
Merge pull request #171 from higepon/master

Revised README.md
Remove jumpapplet.
Added  tips to the Additional Configuration section in README
add option to open file explorer window from the commandline, bump to 21.4.0
update docs about jo(), jc(), jco()
remove _j() as it was never used to begin with.
update docs
minor code cleanup
use zsh change directory hook rather than pre-command hook
handle white space properly when opening with file manager
Update docs on ZSH tab completion.

Closes #178.