Allow user to configure which stock is displayed

[matthew-d-jones]

As a user, I want to be able to choose which stock Casset displays information about.

Acceptance criteria:

• The gui should contain an editable toolbar into which a user can enter a stock symbol (e.g. "AAPL" for Apple)
• When the stock button is clicked, recent information about the stock is displayed on the gui. This should include:
  • The stock symbol for which the information relates
  • The date of the information
  • The close price
• The user-entered symbols should be case-insensitive. E.g. "aapl" and "AAPL" should both return information on Apple
• A clear message should displayed if the stock symbol cannot be found
• A clear message should be displayed if GET requests cannot be made
• Users should be able to continue using the application after successful and unsuccessful requests for information

Points: 3

[TTitcombe]

Closed in #55