Removes pygraph dependency

It was using pygraph for:

- Detect cycles
- Create a dot file based on pipeline's job graph

We had some problems with pyparsing (which is a dependency of pygraph) and then
implemented everything we needed from these libraries:

- Created a method on `PipelineManager` to find cycles (thanks, @gvanrossum)
- Added `PipelineManager.__unicode__` and `PipelineManager.__str__` (returns
  dot representation of Pipeline's graph)
- Added `PipelineManager.save_dot` method, to save a dot file

Pair programming with @andrebco

Fix #42
Adds repr() to `PipelineManager` and `Pipeline`

Also changes `Job.__repr__`
Indents lines inside digraph (DOT language)
Adds `Pipeline.__str__/save_dot` example
Adds comment related to the has_cycle method
Merge branch 'enhancement/remove-pygraph' into develop