Added progress block to uploadFileChunk

As of Dropbox iOS SDK v1.3.5, there is now a progress delegate method for `uploadFileChunk`. So I added a corresponding progress block to the DropBlocks `uploadFileChunk` method.

See SDK changelog:

```
Changes from 1.3.4 to 1.3.5
- Fixed bug where cancel url wasn't always getting sent when the user cancelled
  the authentication flow.
- Fixed a bug where uploads would sometimes fail on iOS 5
- Added a new optional delegate method,
  -[DBRestClient restClient:uploadFileChunkProgress:forFile:offset:fromPath:],
  which allows you to get the progress of chunks you're uploading.
```