Set home directory for libreoffice conversion

Fixes https://github.com/owncloud/documents/issues/291#issuecomment-48953083

I use owncloud's get_temp_dir() because \OCP\Files::tmpFolder(); would change every call. This would be alot of wasted IO + CPU because the 1MB+ user directory would need to be created repeatedly.
@josh4trunks I guess `\OCP\Files::tmpFolder` is better