Merge pull request #4 from dead10ck/master

String IDs, remove methods, docs, unit tests.
Use a MultiDiGraph instead of MultiGraph. MultiGraph is undirected
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.