Support a promise syntax for requests

[kadamwhite]

This adds provisional support for the syntax flexibility specified on the wiki working document:

// All of these should work
postsQuery.get(function( err, data ) {}); // GET, then trigger a callback fn
postsQuery.then(function( data ) {}, function( err ) {}); // Defaults to GET
postsQuery.get().then(function( data ) {}, function( err ) {}); // Specify GET explicitly

It also begins systematically adding doc block comments using YUIDoc, because I find its documentation easier to use than JSDoc's

[kadamwhite]

Forgot a major change: Renamed all of the module constructor methods to use UpperCamelCase naming conventions, so the taxonomies constructor in taxonomies.js became TaxonomiesQuery, wpQuery became WPRequest, etcetera. Now that I think of it, in all of these we should s/Query/Request/g -- I renamed wpQuery to avoid confusion with WordPress's WP_Query object, and since all of the individual endpoint modules extend WPRequest, they should share the same naming scheme.