(NC-142) Allow POST for classification

Since we need to upload a bunch of data in json, allow a client to submit a
POST to classifier/nodes that works the same as a GET but with node data added
in the body.
(NC-142) Merge uploaded facts into node in classification

Merge the data from the POST body into the node that gets supplied to
classification. This required a separate schema for a submitted node, since we
still use nodes without this data internally.
(NC-142) Allow nested lookups in rules

This adds syntax so that the left side of a rule can use nested data. It
preserves the semantics of a single string lookup up a field and adds the
ability to supply a path as a vector of strings.
(NC-142) Update acceptance test to actually submit facts.
(NC-142) Update group docs to reflect changes to the fact syntax in rules

Change existing examples to use paths and add a brief section on what this
means
(NC-142) Flesh out unit test for nested rules and fix schema

When fleshing out the test for submitting facts to the http handler, I found a
bug in the schema for classification/matching-groups which this also fixes.