Fix PROMPT_COMMAND for compatibility with OS X Lion

Fix install.sh to check for sourcing of .bashrc

Instead of updating $PROMPT_COMMAND by appending $AUTOJUMP with a leading
semicolon, prepend $AUTOJUMP followed by a semicolon.

It is unlikely (and frankly wrong) for anything to set $PROMPT_COMMAND to
begin with a semicolon, so this modification should not cause conflicts
where the previous version does.

Use pickle protocol 2.

This ensures that autojump works when the default python interpreter is
changed, e.g. by virtualenv.

Should fix issue #39

autojump is now packaged for Debian too, and will reach Ubuntu.

This should solve
http://github.com/joelthelion/autojump/issues/#issue/34

We are good now except for the shell_utility function which needs to be
refactored.

now that we handle files atomically

(pour les grincheux)

when you "su", the prompt_command can stay set, even without
autojump getting sourced. Then the user's autojump file changes
owner, and a disaster ensues. This is an attempt to fix this
problem.

Courtesy of "Guillaum" on linuxfr:
http://linuxfr.org/~Nonolapero/30339.html

Some people will scream, but XDG_DATA_HOME is broken for now, since
it is not changed when you "su" from a terminal.

When your home is on a network filesytems, multiple shells can try
to write to the autojump store concurrently. The previous system was
a bit dangerous since the name of the temp file was always the same.
This should be better, since the different autojump instances should
all write to different temporaries, and only the atomic rename() should
be concurrent. Time will tell...

don't want autojump to remove paths

Existing files should be migrated automatically