Revert "Use a MultiDiGraph instead of MultiGraph."

This reverts commit 149043bf112e99abb39bb843ab3db0a89afbe555.
Rename SemanticNet.py to Graph.py
Add JSON representation info to README.
Add File System example info to README.
Slight rewording in FS example info.
More compact save_json expression

Reversing the order of the two expressions here makes it
unnecessary to exclude everything I didn't want overwritten.
Fix save_json tests

Test was failing because networkx was reversing the order of the
src and dst nodes. But this is valid in an undirected graph,
so I changed the tests to accommodate this.
Add DiGraph class w/ fixed remove_node()
Add tests for DiGraph
Add networkx_graph() + tests
Fix DiGraph's __init__
Add get_node()
Merge pull request #5 from dead10ck/master

Split Graph and DiGraph