Add a Sample projet

I made a sample project that demonstrates the use of Rebelle for networking code.

It uses the [navitia.io](http://navitia.io/) Open-Data API to fetch a list of places matching a given keyword. There are two variants of the same code, for comparison: one that uses a standard API with completion blocks, and one that uses RBPromise. It demonstrates error handling, chaining, and converting a completion-block-based API to a Promise-based one.