Changes fsSimpleRename to use mv

fsSimpleRename in jiveutil.js uses fs.rename which doesn't work cross-filesystems (see issue #22). This commit changes it to use npm mv (https://www.npmjs.org/package/mv). MV tries fs.rename first and uses fallbacks if it doesn't work.
Merge pull request #31 from 5minpause/fs.rename-to-mv

Changes fsSimpleRename to use mv