First attempt at using the delta API and cursors

@voidlock 
sorry for the whitespace/formatting changes in config.go; I think Sublime did that.  The only real change in there is a new field to track the cursor value.

The delta.go has a function now that will try to use the delta API to only fetch changes since the last run. I didn't change anything in dropbox.go since I know you have a bunch of pending changes in there. All you should need to do is start calling the new 'getFilesToDownload' func to get the []dropbox.Entry slice for stuff that needs to be downloaded.