Add WinINet cache mode, the original mode is named to 'Dedicated'.

See https://github.com/floydpink/CachedImage/issues/3

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.