Add Machiavelli context

"Generic" json feed metric source for [anchor/machiavelli]. Can be altered to
suit any generic json feed.
Update machiavelli metric URL format
Update to newform of metrics endpoint
Pass back raised errors from machiavelli, and empty datasets
Allow a metric name to be passed into machiavelli context
merge machiavelli_context into master, with machiavelli based updates.

from various machiavelli:vendor/assets/javascript/cubism.v1.js updates, see git log
merge the rest of the machiavelli updates
add an error hook to pass any errors from machiavelli to the chart itself.