Changed Initialise in ExcelViewProvider to handle WorkbookOpen

This should resolve issue #12  and #16 

Added event handler for WorkbookOpen.  Taskpanes are currently not registered on opening an existing workbook.  Any check in a ViewModel using VSTOContrib returns a taskpane as null, e.g. visibility this should fix that issue.  Refactored OnNewWorkbook to OnInitialise for consistency.  I didn't find any issues testing it on my side.