Add simple API for parkings

[staszek]

Add simple API for parkings, that allows to get a list of parkings and individual parking details in JSON format.

Things to do:

• The show and index method in ParkingsController should respond to html and json formats.
• The JSON representation of Parking model should consist of id, kind, hour price, day price, number of places, and the address.
• The JSON representation of Address model should consist of id, city, street, ZIP code.
• Add functional test to verify the returned JSON data.
• [Optionally] Exercise the API with a command line tool like curl or wget.