Added an update button (for the user to click when they change the start/end dates)

Updated the code for removing MarketData objects from the ViewModel (used by the chart) to using the RemoveAll with LINQ dynamic predicate technique
Can now download multiple assets at a time. We need this functionality because when the date range is changed, we will re-download the data for all the assets on display.
Changing the date range causes the new data to be downloaded and displayed in the chart.
Merge pull request #13 from farhan85/change-date-range

Change date range