Write a scraper for stock market indices

[AlbertRtk]

Add functionality to get tickers of stocks in a given stock market index, e.g.: get_tickers_in('WIG')

ToDo:

• decide on the output type (list, tuple, DataFrame...)
• function (or class) should be part of marketools.stqscraper
• write a class for Stock Market Index - StockIndex(Stock)