Incremental algorithm

[MeBrei]

Depends on #31

Implement the incremental pagerank approach as suggested by the whitepaper.
The first step should be to further spec out the algorithm and its implementation in the data model document. After that it should be integrated into the osrank_naive implementation. Note that it should be possible to switch between the incremental an non-incremental implementaion.

[MeBrei]

PR for further documentation https://github.com/oscoin/docs/pull/60