Update report's scrollview with better stability. closes #373

Improvements:
- A delay was added (ReportView.cs) before try a call to remote server. In case of fast scrolling, the delay will be canceled.
- The IsLoading state was updated. (SummaryReportsView.cs)
- A better method for calculate current report view. (InfiniteScrollView.cs) 

Fixes #373
Maybe you can just use:

`return visibleViews.FirstOrDefault(v => Math.Abs (pos - v.Frame.X) <= PageWidth / 2);`

?

Just throwing this out there.