Windows-compatibility: New function for OS detection

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 patch introduces a new function -- one that identifies the server's operating system. Several other [patched] modules will make use of this function, including:
*  [Audio Solution Pack](https://github.com/Islandora/islandora_solution_pack_audio/pull/81 "Windows-compatibility: Calling LAME through the command line")
*  [Large Image Solution Pack](https://github.com/Islandora/islandora_solution_pack_large_image/pull/87 "Windows-compatibility: Calling Kakadu through the command line")
*  [FITS Extractor](https://github.com/Islandora/islandora_fits/pull/29 "Windows-compatibility: Windows-compatibility: Calling the FITS processor through the command line")
*  [Islandora OCR](https://github.com/Islandora/islandora_ocr/pull/32 "Windows-compatibility: Tesseract version number")

*NOTE:* This pull request replaces PR https://github.com/Islandora/islandora/pull/441, which became severely messed up.