51 update various classes

[tommyah]

Updated various classes by implementing the following methods:

Portfolio:

• GetNetWorth(): Returns the networth of shares as a BigDecimal

Player:

• GetNetWorth: Returns the networth of shares and money.

Exchange:

• GetGainers(int limit): Gets limit amount of stock winners since latest price change, sorted by highest winner first.
• GetLosers(int limit): Gets limit amount of stock losers since latest price change, sorted by lowest price change first.

Stock:

• GetHistoricalprices(): Returns price history of stock object.
• GetHighestPrice(): Returns highest price
• GetLowestPrice(): Returns lowest price
• GetLatestPriceChange(): Returns latest price change, or 0 if price has not changed.

Also updated the documentation, testing and inlines of various classes.