Separate configuration and instantiation

- Now provides default instance in module
- Create additional instances via (module).create()
- Update tests
Add support for registering partials, helpers

- #registerHelper, #registerPartial, and #registerPartials are
  available on a koa-hbs instance.
- registerPartials scans the specified partialsPath and registers
  all partials found
- No support for async helpers (yet)
Add travis build file