Change to "stat query" model

This currently removes the ability to aggregate stats, but it sounds
like that isn't super important. May add it back in later in the form of
more freeform "stat expressions" (like "cmd_get - cmd_set").

Now instead of sending a list of stats files and buckets, the backend
accepts a list of "stat queries" of the form
`stat_name:flag;option=value` with the k/v options being used to pass
bucket and file, e.g.: `cmd_get:rate;file=node1.zip;bucket=default`

The UI's file and bucket sections now switch the file and bucket to be
used when adding a new stat, but don't switch the stats currently being
displayed, enabling comparing a stat across buckets and nodes.

The current stat queries also can *only* show real data, there is no
longer any interpolation. (rate queries call interpolation, but they
call linear interpolation to get a function to pass to incanter's
derivative function, and only evaluate it at time points that were in
the original data set)

cbugg: closes bug-669

Because there is no longer an opportunity for mape-uppitude

cbugg: closes bug-674

You can now compare across nodes or buckets in the UI

cbugg: closes bug-681
cbugg: closes bug-665

These were bugs in the aggregation code, which has been deleted.