Consistent capitalization of URL

This PR capitializes "URL" consistently:

* All lowercase as a variable name or first part of an identifier (e.g. `url`, `urls`)
* All uppercase elsewhere (second or later parts of an identifier or documentation)
* "Url" is only kept where it is consistent with external standards that use a capital U and lowercase for the rest of the acronym (e.g. KML's styleUrl tag and Bing metadata's imageUrl property)

This makes our use of the URL acronym consistent with our use of the JSON acronym.