Use rolling hash splitter (data sensitive, e.g. Buzhash or Rabin) by default instead of SizeSplitter (data insensitive)

Use the Buzhash or Rabin splitter in IPFS instead of the `SizeSplitter` by default.
A rolling hash based data sensitive splitter has a huge obvious advantage: it creates shift-resistant chunks and thus improves data deduplication and sharing.

For example `casync` is doing it too:
* http://0pointer.net/blog/casync-a-tool-for-distributing-file-system-images.html
* https://github.com/systemd/casync
* https://github.com/folbricht/desync (alternative Go implementation)

Original ticket: https://github.com/ipfs/go-ipfs-chunker/issues/13