Cleanups for `Sanford` module

This does a number of minor cleanups to the `Sanford` module:

* `hosts` is now tracked on the `Sanford` singleton and not as
  a configuration option. Hosts are not configured but are more
  formally registered with Sanford.
* Created a `Hosts` class wrapper that provides the `find` method
  that is used to look up defined hosts. This is more proper and
  provides better testing for this logic.
* Created `register` method on `Sanford`. This is an API method
  for registering a `Host` with `Sanford`. This is explicit
  rather than finding the internals where Sanford stores hosts
  and adding a class directly.
* Consolidated the `config.rb` file into the main `sanford.rb`
  file. The `Config` is so small, it doesn't need another file.