version bump to 0.1.13

Vorher:

```
~/x $ rbenv version
1.9.3-p448 (set by ~/x/.rbenv-version)

~/x $ gem install ~/.rbenv/versions/1.9.3-p0/lib/ruby/gems/1.9.1/cache/screw_server-0.1.12.gem 
Fetching: haml-3.0.13.gem (100%)
haml's executable "sass" conflicts with sass
Overwrite the executable? [yN]  ^C
```

Nachher:
```
~/x $ gem install ../screw_server-0.1.13.gem 
Fetching: json-1.6.5.gem (100%)
Building native extensions.  This could take a while...
Successfully installed json-1.6.5
Fetching: haml-3.1.8.gem (100%)
Successfully installed haml-3.1.8
Fetching: rack-1.1.0.gem (100%)
Successfully installed rack-1.1.0
Fetching: sinatra-1.0.gem (100%)
Successfully installed sinatra-1.0
Fetching: daemons-1.0.10.gem (100%)
Successfully installed daemons-1.0.10
Fetching: eventmachine-0.12.6.gem (100%)
Building native extensions.  This could take a while...
Successfully installed eventmachine-0.12.6
Fetching: thin-1.2.7.gem (100%)
Building native extensions.  This could take a while...
Successfully installed thin-1.2.7
Successfully installed screw_server-0.1.13
8 gems installed
```