Windows-compatibility: Thumbnail generation / Undefined variable (Islandora Paged Content)

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")).

* **Undefined variable:** I believe I've discovered references to an undefined variable, perhaps left over from a previous version of this module. More than once, the module tries to call a variable named "islandora_book_gs," but that doesn't seem to ever get defined by this module or any other. Perhaps this wasn't noticed before because `variable_get` uses a default value ("/usr/bin/gs") when that variable can't be found: a Linux-friendly path to Ghostscript that obviously does not work on a Windows machine. This patch changes the variable name to "islandora_paged_content_gs" instead, which *does* get defined by the module.

* **Thumbnail name:** Windows will sometimes write temporary data to a temp directory rather than to straight memory (this was a [necessary change to the Tuque library](https://github.com/Islandora/tuque/pull/83 "Windows-compatibility: Problems with php:// I/O streams")). As a result, the code that generates the first-page thumbnail needs to be updated. Specifically, the colon in the object ID needs to be replaced with an underscore in the filename, since the [colon is a forbidden character in Windows paths](http://msdn.microsoft.com/en-us/library/windows/desktop/aa365247(v=vs.85).aspx#naming_conventions "Naming Files, Paths, and Namespaces (Windows): Naming Conventions"). Using it results in errors such as: 
`Warning: filesize(): stat failed for <PATH TO TEMP DIRECTORY>/islandora:XX_TN.jpg in CurlConnection->putRequest()`
`The file <PATH TO TEMP DIRECTORY>/islandora:XX_TN.jpg was not deleted, because it does not exist.`

*NOTE 1:* The [Compound Solution Pack](https://github.com/Islandora/islandora_solution_pack_compound/pull/29 "Windows-compatibility: Thumbnail generation (Compound Solution Pack)") is experiencing the same problem.
*NOTE 2:* The function *islandora_deployed_on_windows()* is introduced here: https://github.com/Islandora/islandora/pull/450