adding option `null_as` to treat null values

* null_as defaults to, erm, null, but can be
  overwritten to e.g. 0 (zero)
* also updated README and dashboards.js comments
Update to the lastest rickshaw

Since
https://github.com/shutterstock/rickshaw/commit/417d44847eb750c8e11cc375f39d72a53dc7ba71
min=auto is working properly even if data contains null values.