fix a few bugs and allow setting the y-axis maximum value

Sometimes, graphite will send null as the value of last item in the
datapoints. This results in the graph line going to 0 at the end.
I've worked around the issue by repeating the next to last value, kind of similar to what is already being done in the preProcess method if only one datapoint is received.

Also, when changing between dashboards, the period will change to the default
period, until the next update.
This was fixed by removing "period = default_period", since it has already
been set initially. Maybe there is a specific reason for the line being there?

Lastly, I've made the max option to the rickshaw graph configurable.