Add rambutan microframework

This is another framework that I developed some months ago while teaching to a RailsGirls team how to develop a 'Rails-style' microframework using Rack. The numbers are similar to Nancy and Brooklyn (again, in my machine):

```
rack 6770.40 req/s
cuba 6239.31 req/s
rambutan 5311.18 req/s
brooklyn 5114.88 req/s
nancy 5038.32 req/s
```