Add _last helper function.

JSON data from graphite often contains null as the last value. This
breaks "last" summary pretty often. Taking the last non-null value seems
like a better option.
Add summary_formatter and hover_formatter options

It makes possible to override default _formatBase1024KMGTP formatter
in hover and summary.
Add stroke_width parameter

Now users can override default line thickness (2px) in dashboard.js
Do not replace empty values by zeros

Replacing empty values by zeros is bad practice, and actually not
needed, because recent rickshaw handles empty values just fine.
Roll back to stock rickshaw
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