Api redesign

[curran]

Closes #15

Summary of changes:

• Api expects reactive functions as functions with a property dependencies, an array of strings.
• The library no longer accepts a comma-separated string of dependencies.

Example:

import Topologica from 'topologica';

const fullName = ({firstName, lastName}) => `${firstName} ${lastName}`;
fullName.dependencies = ['firstName', 'lastName'];

const state = Topologica({ fullName });

[monfera]

comment added to https://github.com/datavis-tech/topologica/issues/15#issuecomment-416581831