[Graph]: generate plain text files for .Rd files properly.
[Graph]: Add documentation in plain text for FlashGraphR.
[Graph]: extract an induced subgraph.
[Graph]: add sparse matrix dense matrix multiplication in FlashGraphR.
[Graph]: check the vector length in SpMV in FlashGraphR.
[Graph]: FlashGraphR compress subgraphs before giving it to igraph.
[Graph]: fix a bug in FlashGraphR.

If two graphs have the same name, we should use the new graph to replace
the old graph in the graph table.
[Graph]: remove time measuring in release test vs. igraph.
[Graph]: fg.degree gets #in-edges and #out-edges as well.