Weaken restrictions on Surveyor timestamps.

NCS Core uses the json-schema Ruby library[0], which incorrectly
interprets schemas like this:

    sub.json
    --------
    { "attr": {
        "extends": {
          "$ref": "http://www.example.edu/super.json"
        },
        "type": [
          "string",
          "null"
        ]
    }}

    super.json
    ----------

    { "type": "string" }

Section 5.26 (on "extends") of JSON Schema Draft 03 states

    A schema that extends another schema MAY define additional
    attributes, constrain existing attributes, or add other constraints.

However, when validating a document like

    { "attr": null }

against sub.json, the json-schema library will incorrectly flag it as an
error:

    The property '#/' of type NilClass did not match the following type:
    string

I generally find altering specifications to satisfy implementations to
be distastefully backwards, but in this case I'll make an exception:

1. We need NCS Core's sync system up as fast as possible.  This is the
   most expedient path.
2. I have no evidence to prove this -- so this is very much a
   ready-fire-aim deal -- but I doubt anyone else is doing anything
   serious with Surveyor's JSON representations and a JSON Schema
   validator.

The right way to fix this would be to use an up-to-date draft of JSON
Schema (or adapt some other means of communicating JSON formats), but
draft 04 is still unpublished, and JSON Schema is the leading effort in
this field.  I guess we could also switch to a different validation
engine for NCS Core, but that takes more effort than just making
timestamps nullable all around.  So there you go.

[0] https://github.com/hoxworth/json-schema

doc/response_set_schema.json

@@ -3,13 +3,7 @@
     "description": "Response set",
     "properties": {
         "completed_at": {
-            "extends": {
-                "$ref": "http://download.nubic.northwestern.edu/surveyor/surveyor_timestamp_schema.json#"
-            },
-            "type": [
-                "string",
-                "null"
-            ]
+            "$ref": "http://download.nubic.northwestern.edu/surveyor/surveyor_timestamp_schema.json#"
         },
         "created_at": {
             "$ref": "http://download.nubic.northwestern.edu/surveyor/surveyor_timestamp_schema.json#"

doc/surveyor_timestamp_schema.json

@@ -2,5 +2,8 @@
     "$schema": "http://json-schema.org/draft-03/schema#",
     "description": "Timestamp on a Surveyor entity",
     "format": "\\d{4}-\\d{2}-\\d{2}T\\d{2}:\\d{2}:\\d{2}(?:Z|[+-]\\d{2}:\\d{2})",
-    "type": "string"
+    "type": [
+        "string",
+        "null"
+    ]
 }