Add TODO about the arrivals reponse

Issue #3.
Store the session cookies

- Choosed MongoDB, but tried to keep it implementation agnostic.
- Implemented SessionCookie::Storage.
- Modified the attribute hash of SessionCookie to have strings as keys. That
  was because the MongoDB driver uses them when finding a document.

Issue #2.
Store stops

In order to store stops we have to ask for the routes of every bus line. I
wasn't confortable with scraping the ids and names, so I hard-coded them
(in Stop::Obtainer::BUS_LINES).

Added classes to:
  - Obtain all the stops (Stop::Obtainer)
  - Merge the duplicate stop information (Stop::Collection)
  - Represent the routes query (Route::Query)
  - Transform the routes response into something useful (Route::Parser)

Also added shortcuts methods to:
  - Query the bus line route (Route.query)
  - Update the stops in the database (Stop.update_db)

Issue #8.