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")).

This viewer caches the JP2 file in the tomcat temp directory. However, in Windows, the path to this file contains backslashes, which jQuery is mistaking as 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.

*NOTE:* There may be a more efficient way to make this path conversion happen.