Refactored dependencies into a class and added short-circuiting for condition
dependencies that have already been verified to be broken.
Added shot-circuiting for well-defined conditions that have already been
verified.
Added better short-circuiting on well-defined conditions: a cond's tree should
only need to be traversed one time now.
Added the fail_on_undefined_vars flag to 'exists' conditions.
Changed 'bad condition' msg to 'ill-defined condition'
Tiny edits to display_paths in reporter class.
Added a bit of formatting to the Reporter class and made display_paths a
Reporter method.
Added support for XOR conditions to dependency grapher.
PEP8 formatting edits.
Merge pull request #12 from FranciscoCanas/dep_path_shortcircuit

Dependency Tests Refactoring