Windows-compatibility: Calling Kakadu through the command line

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 Kakadu is being called through the command line, some of the syntax is very Linux-specific and fails under Windows. This patch ensures the appropriate syntax is used based on the server's operating system.

To elaborate: 

> `command -v kdu_compress >/dev/null 2>&1` contains some Windows-incompatibilities. To make it more Windows-friendly, "command -v" should go away. Also, "/dev/null" should be replaced with "NUL", which is the equivalent Null Device Driver in Windows. As a result, the Windows equivalent of that command is: `kdu_compress >NUL 2>&1`

*NOTE:* The function *islandora_os_check()* is introduced here: https://github.com/Islandora/islandora/pull/450