Specs: make element registrations be per-module, define how they are

sky/specs/modules.md

 Sky Module System
 =================
 
 This document describes the Sky module system.
 
 Overview
 --------
 
 The Sky module system is based on the ```import``` element. In its
 most basic form, you import a module as follows:
 
 ```html
 <import src="path/to/module.sky" />
 ```
 
 As these ```import``` elements are inserted into a document, the
 document's list of outstanding dependencies grows. When an imported
 module completes, it is removed from the document's list of
 outstanding dependencies.
 
 Before executing any ```script``` elements, the parser waits until the
 list of outstanding dependencies is empty. After the parser has
 finished parsing, the document waits until its list of outstanding
 dependencies is empty before the module it represents is marked
 complete.
 
+
 Module API
 ----------
 
 Within a script in a module, the ```module``` identifier is bound to
 the ```Module``` object that represents the module:
 
 ```javascript
 interface Module : EventTarget {
   constructor (Application application, Document document); // O(1)
   attribute any exports; // O(1) // defaults to the module's document
   readonly attribute Document document; // O(1) // the module's document
   readonly attribute Application application; // O(1)
 }
 ```
 
 ### Exporting values ###
 
 A module can export a value by assigning the ```exports``` property of
 its ```Module``` object. By default, the ```exports``` property of a
 ```Module``` is its ```Document``` object, so that a script-less
 import is still useful (it exposes its contents, e.g. templates that
 the import might have been written to provide).
 
+### Exporting element definitions ###
+
+When importing a module into another, Sky looks at the properties on
+the imported module's ``exports`` value, and for each property that is
+an element constructor (generated by ``registerElement()``), it adds
+an element to the importee's element registry.
+
+ 
 Naming modules
 --------------
 
 The ```as``` attribute on the ```import``` element binds a name to the
 imported module:
 
 ```html
 <import src="path/to/chocolate.sky" as="chocolate" />
 ```
 
 The parser executes the contents of script elements inside a module as
 if they were executed as follow:
 
 ```javascript
 (new Function(name_1, ..., name_n, module, source_code)).call(
   value_1, ..., value_n, source_module);
 ```
 
 Where ```name_1``` through ```name_n``` are the names bound to the
 various named imports in the script element's document,
 ```source_code``` is the text content of the script element,
 ```source_module`` is the ```Module``` object of the script element's
 module, and ```value_1``` through ```value_n``` are the values
 exported by the various named imports in the script element's
 document.
 
 When an import fails to load, the ```as``` name for the import gets
 bound to ```undefined```.