Update various classes

[tommyah]

This task is dedicated to the process of updating multiple classes to satisfy the requirements given in the second part of the Millions project description.

This task includes:

• Updating stock class with getter methods for historical prices (all prices), lowest price, highest price, and latest price change (0 if first price)
• Updating exchange class with methods for getting gainers and losers on the exchange this week.
• Update portfolio class with method for getting networth, including the effective value of owned shares.
• Update player class with method for getting networth, including the effective value of owned shares.