hide push/popd message. make git pull working with older git version
use subshell and not push/popd
fix argument order
add a selfupdate function to update antigen itself
Updated cram to the latest.
Merge branch 'master' of https://github.com/xgarrido/antigen
Avoid git cd-less magic.

It appears that the whole git magic that's going on to avoid starting a subshell
is causing problems on certain slightly old versions of git. Now switched to
`cd`-ing to the clone to avoid any trouble in this regard.
Moved `-set-default` into a private function.

Its polluting the global namespace but doesn't have the `antigen` prefix, making
it hard to figure out where it came from.
Refactorings on the selfupdate function.
Add readme notes for `-selfupdate` command.