Remove pygraph dependency

[turicas]

Currently we use pygraph for:

• Verify if a pipeline has a cycle
• Generate a .dot file (to plot the pipeline using graphviz)

But it depends on some big packages, like pyparsing, and the new version of it (pyparsing) has some bugs when installing through pip.

Probably we can implement by hand the features of pydot so the whole pypelinin package (with all its dependencies) will be lighter.