add DELETE_LOCAL_FILE conf

add config of DELETE_LOCAL_FILE, with DELETE_LOCAL_FILE=False, it will not delete local file when file not in Dropbox.

If you have a sync folder named test. When without network, it will not upload to Dropbox. When you connected to network, Drop2Pi will found this folder is not in server. 0.02 version will delete local file or folder, because we think the server version always right. But that will delete the local file you did not if is uploaded.

so with DELETE_LOCAL_FILE=False, it will save your local file.

a little change.