Merge pull request #6 from seatme/master

Add MFA support to the keychain util
bump version to 0.0.9
Run one command with AWS env vars set

If more arguments are passed to aws-creds shell, it'll run the given
command rather than spawning a sub-shell.
exec rather than system, plays better with unix signals and pipes
Merge pull request #7 from codekitchen/shell_command

Run one command with AWS env vars set