put big algorithms in separate module

[mhelvens]

In order to keep the core graph.js library light, larger algorithms like .paths(), .cycles(), .transitiveReduction() and .contractPaths() should be put in a separate companion library.

This is a breaking change that will wait for version 2.x.

[dominicbarnes]

In addition to extracting these algorithms, personally I would find it helpful to have better descriptions for what they do. I'm fairly new to graphs as a whole, so some of the terminology makes it hard to grok the overall concept.

I'm thinking that having separate libraries would make it easier to dedicate time to explaining the concepts and providing some example use-cases.