Fixed unreliability with IE < 9

Using embedded images (for the route plan image) as Base64 is
sophisticated, yes, but the IE in versions earlier than 9 would (if at
all) only support 32 kB of such data! This renders it unusable, since
the .Net Framework uses the IE for its WebBrowser control.

Solution: Don't use Base64-embedded image. Instead, store the route
image as a temporary file and include the full file path to this file
inside the HTML. This works far better. Also, remove the file after it
has been used.