Add WinINet cache mode, the original mode is named to 'Dedicated'.
Code cleanup by ReSharper.
Improve image download progress to avoid write-write confliction when several requests requesting the same image simultaneously.
Directly provide MemoryStream to BitmapImage instead download image to memory, write file and read file into the memory again.
Update README.md.
Merge pull request #5 from stackia/master

Added an AppCacheMode property to use the current file system based caching or the new (default) IE-temporary files based caching.