Various fixes for UBER spec

This branch contains a number of spec fixes based on feedback in Issues and online. Below is a summary of the changes.

 * Change title to "Uniform Basis for Exchanging Represenations (UBER)"
 * Change "Uber" to "UBER" throughout document
 * Change "diff" to "partial", "update" to "replace"
 * Change "send-format" to "sending", "receive-format" to "accepting"
 * In JSON, make "rel", "sending", and "accepting" arrays
 * Add Normative refs to RFC2616 and RFC5789
 * Add Informative ref to CoAP-18
 * several minor editorial fixes