Cleanup launcher

This is mainly a refactoring branch, but there is one __breaking change__ (from dee09a9).  You will need to change this:

```ruby
driver :foo, Bar do
  # ...
end

server :baz, Shpee do
  # ...
end

enable_driver :foo
enable_server :baz
```

to this:

```ruby
drivers do
  configure :foo, Bar do
    # ...
  end
  enable :foo
end

servers do
  configure :baz, Shpee do
    # ...
  end
  enable :baz
end
```

This should help group things together, make the similarities between server and driver config more obvious, and it reflects the internal structure in which the drivers/servers are now stored.