Add remote file existence checking

Upyun limits file listing to 10000 at most. This limitation is likely to be broken under real-world-production circumstance. When incrementally sync a large asset pool with upyun, most time will be spent on waiting for asset downloading. Thus, I add this remote existence checking to boost the sync speed.
This feature is also required for the upyun incremental sync tool I'm working on: https://github.com/yi/upyun-assets-syncer