Fix running bin/shoes.bat from source

Fixes #918.

On gem installation, we copy `ext/install/shoes.bat` over to the Ruby bin
directory. The batch file assumed that it would be located next to
`ruby-shoes` to execute.

When running from source, though, we were "linking" `bin/shoes.bat` to just
point to the `ext/install/shoes.bat` location. On *nix systems, the symlink
would behave as if it was in the bin directory, but on Windows the file
is literally just a single line reading `"ext\install\shoes.bat"`, which
executes the ext copy of shoes.bat, but from the wrong location.

The fix here was to make `ext/install/shoes.bat` take a `bin_dir` if already
set, no longer treat `bin/shoes.bat` as a link to that file, and have
`bin/shoes` properly set the `bin_dir` so running `ext/install/shoes.bat` works.

Fun times in trying to not have your Ruby program get started by Ruby :)

cc/ @glenn-murray-bse in case you had bandwidth to test this out on your
Windows machine. Worked fine for me, but since I'm primarily on Mac,
would appreciate a double-check.