Added mechanism for downloading sample data (similar to Store).

- Added Sample data view model
- Added MainPage button with flyout for download UI
- Added RequiresLocalData property for each sample (gleaned from <localdata> element in xdoc comment)
- Added Local Tiled Layer sample
Merge branch 'master' into greg/DownloadLocalData
Informational text changes.
Merge pull request #31 from Esri/greg/DownloadLocalData

Sample data download functionality