Move objgraph.py to objgraph/__init__.py

[klahnakoski]

In light of #39 I would like to start a refactoring: The first step is to move objgraph.py to objgraph/__init__.py. This will establish a code directory that is separate from all the deployment resources found in the main directory, and it will be where we add further (graph) modules, and maybe even break up the main code file.