add missing rack namespace

`Builder`, `Request`, `Handler`, `Lock`, `Session::Cookie` were missing Rack's namespace. 

This should fix the issue mentioned in #15.

After this change, I'm getting the error:
```
ERROR RuntimeError: missing run or map statement
/home/hrvoje/.rvm/gems/ruby-1.9.3-p385/gems/rack-1.5.2/lib/rack/builder.rb:133:in `to_app'
/home/hrvoje/.rvm/gems/ruby-1.9.3-p385/gems/rack-1.5.2/lib/rack/builder.rb:138:in `call'
/home/hrvoje/.rvm/gems/ruby-1.9.3-p385/gems/rack-1.5.2/lib/rack/handler/webrick.rb:60:in `service'
/home/hrvoje/.rvm/rubies/ruby-1.9.3-p385/lib/ruby/1.9.1/webrick/httpserver.rb:138:in `service'
/home/hrvoje/.rvm/rubies/ruby-1.9.3-p385/lib/ruby/1.9.1/webrick/httpserver.rb:94:in `run'
/home/hrvoje/.rvm/rubies/ruby-1.9.3-p385/lib/ruby/1.9.1/webrick/server.rb:191:in `block in start_thread'

```

Will investigate further.