Fix white select area in windows

[Marc-9]

CLOSES #5, #82, #142

When running Excalibur in windows, selecting the area where the tables exist whether by your mouse or by autodetect tables shows a white box rather than the document behind it.
Looking at your terminal shows this is because of a 404 as the server can not find the image to show.
For some reason that I have not figured out on windows only, the image path gets mangled and some of the forward slashes are instead replaces with backslashes.

This resolves that issue.

[Marc-9]

I realized that replacing the backslash problem on the server solved the issue for pdfs that were already uploaded but any new pdfs uploaded would not work.

By making the chance occur when the select area library tried to access it seems to fix the problem for previous uploads and future uploads.