enhancement: tarsnap chunker / importer (Content defined, polynomial, fast converging chunking)

#### Version information:
go-ipfs version: 0.4.4

#### Type: Feature, Enhancement

#### Priority: P4

#### Area: Tools, Importer

#### Description:
Better suited for maximizing deduplication ratio then current Rabin chunker.
Using smaller chunks with faster convergence yields greater space savings, and the benefit depending on dataset can be great in comparison to Rabin.

The mean chunk size used by tarsnap is 64k.

Source:
https://github.com/Tarsnap/tarsnap/blob/master/tar/multitape/chunkify.h
https://github.com/Tarsnap/tarsnap/blob/master/tar/multitape/chunkify.c

Related:
https://moinakg.wordpress.com/2012/11/11/inside-content-defined-chunking-in-pcompress/
https://moinakg.wordpress.com/2012/11/15/inside-content-defined-chunking-in-pcompress-part-2/