Adding concurrent download capabilities.

I mostly followed along with this paper and modelled the walking and
processing after the bounded.go example.

http://blog.golang.org/pipelines
Merging in multi download changes.
Merge pull request #7 from articulate/multi_download

Adding concurrent download capabilities.
First attempt at using the delta API and cursors to only fetch the files that have recently changed or been added.
using new error handler function