Add 'json-array' format

This provides an array of (x, y1, y2) tuples
instead of multiple arrays of (x, y1), (x, y2)
tuples.  It is efficient (the timestamp
is not repeated) and convenient for use with
the dygraphs library.
Enable serving graphite.dashboard app outside root

Previously, serving dashboard app from anywhere but
/dashboard/ would break things because there
are hardcoded absolute paths in dashboard.js.

This change adds settings.GRAPHITE_API_PREFIX, to
allow use of the dashboard app at some other place
in the URL namespace.

This is useful if we want to deploy graphite along with
other WSGI applications, and use a /graphite/ prefix
to namespace it away from other apps.
Fix json-array rendering when no data is found