Partially fixed #649342: Index theming can now be overriden by themes not only with a <code>index.tpl.php</code> file, but also with <code>index--$iid.tpl.php</code>.

Every context can have a path now. If it is set, it will be used to create a link from the context name at admin/build/index/list.

Cleaned up some cruft. Decoupled code for page and block contexts from 'core' functions to clean up the code and provide examples for other developers on how to extend Index.

Fixed bug that caused Taxonomy and Comment entity types to be defined, even though those modules weren't enabled.

Fixed bug that would cause entities to show up more than once when repeatedly calling view() on the exact same index object.