don't want autojump to remove paths

Existing files should be migrated automatically

Change the way we keep the database under a reasonable size, to better support removable drives
Instead of removing non-existent paths, keep the 300 more recent directories

were incorrect. Removed those comments.

install.sh:
1. for those who have a /etc/profile.d
   then grep .bashrc for the sourcing of the autocomplete code
2. for those who use OSX
   grep .bash_profile to make sure .bashrc is sourced.

Rationale: regexes in the dirspec break autojump for weird directory
names like "c++". In more than one year of use, I've never even been
tempted to use a regexp with autojump. So I think it is safe to remove
this feature. If enough users shout, I will revert this change.

in order for newline characters to be parsed correctly

rationale: if a user's directory isn't writeable, then autojump is
probably his last concern :)

Leaking vars into the user's zsh env is not good, esp. when the var
refs a directory -- zsh may then interpret it as a named directory
ref and use it in prompts, for e.g.

Autojump is really starting to mature :)