Windows-compatibility: Backslashes in paths (OpenSeadragon)

Some Windows-incompatibilities have crept into the Islandora codebase ([additional background can be found here](https://groups.google.com/d/msg/islandora/0MAhLDjbago/9knbOynlyIcJ "Islandora 7 and Windows: Patches and Tips")).

When the page first loads, this viewer caches the JP2 file in the tomcat temp directory. However, in Windows, the path to this file contains backslashes, which jQuery mistakes for escaped characters. As a result, the viewer loads, but the JP2 image never loads inside of it. This patch converts all the backslashes in the path to forwardslashes, which fixes the problem.