No check for existence of sudo command

[troeger]

Cusine (and Fabric ?) silently assume the 'sudo' command to be available on the machine, which is not given on some minimal Debian setups.

I would propose to add an according check in the initialization step, resulting in a warning. Doing such a check in the sudo() method would indirectly break some package managers, which might be confusing for users.

If needed, I can provide a patch for that.

[sebastien]

This is a very good idea. Also, it would be good to check that the shell is SH-compatible, as it's a requirement for Cuisine to work. Looking forward to your pull request!