Remove '/' request path requirement for flexibility

This allows you to do things like:

```ruby
# config.ru
require 'my/service'
require 'stark/rack'

map '/foo' do
  use Stark::Rack
  use My::Service::Foo::Processor
  run My::Service.new
end

map '/bar' do
  use Stark::Rack
  use My::Service::Bar::Processor
  run My::Service.new
end
```