Vendorize argparse

This adds the argparse library [1] as autojump_argparse and imports it
via sys.path mangling in case argparse does not exist (Python 2.6 or
below without argparse installed).

This makes autojump effectively work with Python 2.6 again by default.

I have not verified license compatibility, but given the intention of
the (backport) project this is likely OK (it is licensed under the
Python license).

1: http://code.google.com/p/argparse/

Fixes issue #121.
Merge pull request #150 from blueyed/master

Make autojump work with Python 2.6 again: vendorize argparse.
version numbering
Fix #148, add documentation regarding PROMPT_COMMAND. Bump to v21.0.1
Update install.md: Python 2.6 is supported again
set permission 0755 to _j in install.sh
minor documentation reformat
minor Makefile edit
minor ChangeLog update
Add autojump to the end of PROMPT_COMMAND
version bump to v21.0.3
remove adding git version to autojump install
Test for brew existence before testing for brew zsh site functions directory.

The previous test condition would cause errors because even though brew didn't
exist, the directory /share/zsh/site-functions did and it would throw error
messages on non-homebrew systems.
Update README.md
Merge pull request #157 from barsch/patch-1

Minor formatting update for README.md
Update README.md

The recommendation doesn't work with bash 4.1.5, the code in autojump.bash is fine, so use it here.
Merge pull request #158 from megies/patch-1

Update README.md with correct PROMPT_COMMAND.
Reverts commit 74c50133d2. Fixes #159, #160.

Even though the previous version works fine on OS X 10.6.8, it seems to be
causing problems for other Mac users on 10.8.x. Unfortunately I do not have
access to a newer Mac to test on.
refactor a few lines for legibility
Consolidate version numbers.

Instead of keeping the version number in two separate places, the Makefile will
now grep bin/autojump for the proper version number.