Change client to use proper flags and subcommands parsing.

I rewrote all the cli in order to have a proper basis and to rely on the existing library `github.com/codegangsta/cli`.

It results in the following improvements :
* code prepared for internationalization (less strings anywhere in the code, internationalization could be implemented right in `github.com/codegangsta/cli`),
* better settings management,
* better error handling (exemple in `lxc/commands/ping.go`),
* better code organization and modularity (according to me),
* and finally, ability to enable bash completion.

*I also added a vim modeline to avoid any issue related to code indentation in vim.*