Windows-compatibility: Calling LAME 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 LAME is called through the command line, the quotes around the filenames cause the command to fail. The following code corrects that.

*NOTE:* The function *islandora_os_check()* is introduced here: https://github.com/Islandora/islandora/pull/441