bin/autojump.zsh: only add existing dirs

With a custom install method (e.g. checkout in ~/.autojump), ~/.autojump/functions will not be present.
Makefile: prepend `./` to (un)install.py
Merge remote-tracking branch 'blueyed/autojump.zsh-only-add-existing-dirs'
Fix `Needles must be a list error` for `foo_1`

Fixes https://github.com/joelthelion/autojump/issues/274
Fix regression with `j foo__`

With `j foo__` the first index is assumed now again.

Fixes https://github.com/joelthelion/autojump/issues/271
Escape backslashes in regexps (match_anywhere/match_consecutive)

Fixes https://github.com/joelthelion/autojump/issues/281
Fix `is_cwd` for symlinks

Since `pwd` is a resolved symlink, the comparison in `is_cwd` needs to
use the real path, too.
README: minor fixes