Windows-compatibility: Complex syntax problem

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")).

**Typo?** I believe I discovered a typo -- or perhaps a relic left over from a previous version of this module. Twice, the module tries to call a variable named 'islandora_book_gs', but that doesn't seem to exist. This patch changes the variable name to 'islandora_paged_content_gs' instead, which does get defined by the module.

**Complex syntax:** For some reason, when complex syntax {$...} is used, the code inside the curly braces sometimes doesn't get processed, resulting in error messages like: `"The file {$page->id}_TN.{$ext} cannot be found."` I'm not sure why this is happening, but concatenating the variables instead seems to fix this problem -- and should work with both Linux and Windows.