Support :Port as string

This patch fixes an issue with running `rackup --port 3333`. For some reason `rackup` doesn't convert the argument to an int before passing it to the handler, and currently Jubilee's `RubyServer` expects this option to be numeric.

An example:

```
% cd spec/apps/rack/basic
% ruby -I ../../../../lib -S rackup -s jubilee --port 3333
TypeError: can't convert String into Integer
  initialize at org/jruby/jubilee/RubyServer.java:69
  initialize at /Users/theo/Documents/Code/jubilee/lib/jubilee/server.rb:9
         run at /Users/theo/Documents/Code/jubilee/lib/rack/handler/jubilee.rb:24
       start at /Users/theo/.rvm/gems/jruby-1.7.8@jubilee/gems/rack-1.5.2/lib/rack/server.rb:264
       start at /Users/theo/.rvm/gems/jruby-1.7.8@jubilee/gems/rack-1.5.2/lib/rack/server.rb:141
      (root) at /Users/theo/.rvm/gems/jruby-1.7.8@jubilee/gems/rack-1.5.2/bin/rackup:4
        load at org/jruby/RubyKernel.java:1103
      (root) at /Users/theo/.rvm/gems/jruby-1.7.8@jubilee/bin/rackup:23
```

But with this patch it starts nicely.

Thanks for Jubilee, I like it a lot. I hope I can help out along the way.