Handle the case where grep is an alias to egrep same way as autoenv_check_authz() do.

egrep -G (grep -EG) results in conflicting matchers
Fix shasum not being available on all distros. Fixes #23

Changed shasum to sha1sum.
Update README.rst
Merge pull request #47 from z4y4ts/patch-1

Handle the case where grep is an alias to egrep same way as autoenv_check_authz() do
Should now work under OS X
Merge pull request #55 from mahmoudhossam/patch-1

Fixed problem with OS X not having sha1sum
Change check for shasum

shasum wasn't getting found on OS X 10.9.1 with zsh 5.0.5
Update README.rst with required Mac Homebrew step

After googling to https://github.com/kennethreitz/autoenv/issues/41 and https://github.com/kennethreitz/autoenv/pull/51 I finally did this to make autoenv work.
Merge pull request #60 from groovecoder/master

Update README.rst with required Mac Homebrew step
Merge pull request #58 from unixorn/master

Change check for shasum
Use `chpwd_functions` array in zsh.

This is a better way to load autoenv than overriding `cd` in zsh, because with
this method, autoenv will work fine even if auto_cd option is turned on in zsh
(typing just the directory name without a `cd` to change directory).

And (as a minor point) it produces cleaner error message,

autoenv_cd:cd:2: no such file or directory: non-existent
vs.
cd: no such file or directory: non-existent
Add notes in README about loading on zsh.
Add ability to run commands when exiting a directory.