Tidy up some rough edges.

* Schemas are validated upon retrieval. Previously we only verified that the
  schema decodes to a dict, and trusted that the server will not allow invalid
  schemas to save.
* Consequently, 'http_get_schema' now raises jsonschema.SchemaError exceptions
  rather than ValueError. Updated test to reflect this and added local binding
  for SchemaError.
* Switch from using 'unquote_plus' to 'unquote'; we're not encoding form fields
  so the '+' = space thing doesn't apply.
* Added a helper, 'ncsa_utcnow', which produces a string timestamp of the
  current time (UTC) in NCSA Common Log Format. Needed by el-devserver.
* Replace dependency on 'zmq.util.json' with our own ranking (ujson, than
  json). The simplejson in CPython 2.7's standard lib (i.e., 'json') includes
  the C extension, so it's already quite fast. Compared to alternatives, only
  ujson is significantly faster.

Change-Id: Id146ea42de6e3cc709514d5787a3dc76a2362654