J nery/issue35 measure completed graphic

This resolves #35 

@hmassih  : Please code-review
Changes:
- (MeasureDisplayControl.xaml.cs) Allow MeasureCompletedEventArgs.Geometry be result of RequestShape regardless if polygon formed. Suspend clearing of graphics and display until first vertex is committed.
- (MapViewModel.cs) Removed renderer from MeasureLayer and instead assigned symbol on graphics so when MeasureCompleted is handled, vertices and polyline can also be added to layer.