Add method to parse the offers

[ivysochyn]

Currently, the only thing the parser is doing - is extracting offers from a search page into a database.

The task is to add a function to follow the offer URL and extract:

• Price + additional payments
• Apartment size (in square meters)
• Amount of rooms
• Floor
• Description

[ivysochyn]

I started working on the parser function and successfully implemented a baseline for OLX website, the otodom is to be done.

Currently, all the meta-data isn't extracted and converted to the proper type, but stored as a raw string. This should be fixed later.

Also, additional refactoring would be useful in order to reduce the complexity of a parsing function and choose proper names for structures' attributes.

[ivysochyn]

Added functions to parse both olx and otodom offers and merged changes in #18