Refactored how files are updated to ensure files are transferred in
accordance with the rsync specification with regards to the --update
flag.  Now files are only skipped if the update flag is set and the
destination file is newer.  If the destination file is the same or
older, then the file is transferred regardless.  If update flag is not
specified, then the file size / checksum, etc. are considered.
Implemented the checksum option.
Merge branch 'master' of https://github.com/iwonbigbro/gsync