sudoers file: add default secure_path

On Wheezy, secure_path is not set by default anymore (was set to a
hardcoded value in Squeeze). To make running commands without path
working again in Wheezy, we want to set a standard PATH value by
default.
Merge pull request #24 from ckaenzig/set-defaults-secure-path

sudoers file: add default secure_path