Minor changes to ease initial project setup

Three commits in this pull request:

* Additional `require` statements are needed in order to invoke the _Servitude.boot_called_ attribute accessors
* The readme doesn’t mention that _Servitude::Configuration_ must be mixed in, and it’s possible to imagine a server that needs no configuration, so I think it makes sense for the mixin to be truly optional
* When you provide no command-line arguments, you see `unknown command ""` — I think it makes sense to avoid this