Integration of the HDRF algorithm within PowerGraph.

HDRF is a novel stream-based graph partitioning algorithm that provides important performance improvements with respect to existing solutions in partitioning quality.
In particular, HDRF provides the smallest average replication factor with close to optimal load balance. These two characteristics put together allow HDRF to significantly reduce the time needed to perform computation on graphs and makes it the best choice for partitioning graph data.

The HDRF algorithm is extensively described in the following publication:

F. Petroni, L. Querzoni, K. Daudjee, S. Kamali and G. Iacoboni: 
"HDRF: Stream-Based Partitioning for Power-Law Graphs". 
CIKM, 2015.
http://www.dis.uniroma1.it/~midlab/articoli/PQDKI15CIKM.pdf

src/graphlab/graph/distributed_graph.hpp

@@ -191,7 +191,7 @@ namespace graphlab {
    *                    runtime memory consumption.
    *
    * \li \c "grid" Runs at rouphly the same speed of random. Randomly places
-   *                edges on machines with a grid constraint.–
+   *                edges on machines with a grid constraint.
    *                This obtains quality partition, close to oblivious,
    *                but currently only works with perfect square number of machines.
    *