Update current price of the stock in the portfolio and implement ability to sell shares

When a user goes to their portfolio, the current price of the stock is updated through a call to to Yahoo Finance.  Currently only the user's stocks are updated but a later implementation may explore updating all the stocks in the Stock table at a regular interval (every 15 min?).

Implemented ability to sell shares.  User can sell shares from the portfolio page.  Still need to delete stocks from user's portfolio if the user has zero shares.

Updated flash messages to use a category and deleted any instances of passing error messages to templates.

Centered text in td elements.