#3 Checksum file is added to checksum

[Ben]

A minor bug I've noticed, apologies if you already know
about it.

This is using WxChecksums in windows. If I create a new
checksum file (by right clicking in explorer) and add
the contents of the current directory (as I usually
do), the checksum file itself is added. This obviously
isn't particularly useful, especially as that file
changes when I save it, making the checksum incorrect!
Perhaps the program, when adding files to a checksum,
could ensure that it doesn't add the checksum file itself.

Many thanks for a very useful program,

Ben