Adds url-as-a-graph feature to dashboards

Before this commit, when writing a dashboard and including a graph in
it, we had to:

1. create a datasource metadata file on top of a datafile
2. create a graph metadata file accessing that datasource

But most of the time we just wanted to graph all available columns from
a datafile.  This change allows one to simply specify a datafile url
instead of a graph id.  Limn fetches the datafile, infers a datasource
from it, and builds a graph of all the metrics in the datasource.
Caveat: DO NOT add multiple datafiles with the same filename to the same
dashboard, unless they're exactly the same (because Limn will cache the
datasource and serve it both times).
Merge branch 'develop' of github.com:milimetric/limn into develop

Conflicts:
	src/graph/graph-view.co