refactor(index): provide a cleaner API surface

BREAKING CHANGE: The public API to Dgeni has changed:

Previously the main index module exported a factory function that returned
a `generateDocs` function.  Now it exports an object containing:

```
{
  log : // The winston logging object
  Config: // The class to instantiate when providing a config object to Dgeni
  loadConfig: // A function to load a config file
  generator: // The main doc generator factory function
}
```
chore(dgeni.conf.js): add a simple dgeni config