Add pickle_env.py

Tool for generating pickle files used by tests
Add test_render_tree_cyclic_dependency

Test for issue in #13 (https://github.com/naiquevin/pipdeptree/issues/13)
Fix infinite recursion for cyclic deps

Fixes #13 (https://github.com/naiquevin/pipdeptree/issues/13)
Merge pull request #14 from msabramo/master_fix_infinite_recursion

Fix infinite recursion for cyclic deps (#13)