beta version of filter

[wweic]

how to test:

make mgraph_test
./mgraph_test c                                   # create graph
./mgraph_test l test_graph                        # show original graph
./mgraph_test f                                   # filter original graph
./mgraph_test l test_graph_transformed            # show filtered graph

[wweic]

Fix issue #41

[thinxer]

1. Why move mgraph.hpp into io? graph.hpp may act as the main header for the graph package and hide implementation things like mgraph.
2. The interface looks good.
3. We may need another graph builder for this kind of graph creation, which should write the vertex and edge data directly to file, without a cache in memory. However, this can be delayed as it's a performance booster not a functionality enhancement.

[wweic]

1. I think all files with same function should live in a folder, the hiding job can be done by sae_include.hpp. Well Maybe in outer directory is more appropriate, subdirectory is for internal implementation.
2. another branch will do this job.