Doc browser and media type collection improvements

* doc browser
 * Avoid showing certain internal type options (i.e. reference).
 * Fix Type label to detect collections better, and differentiate between attributor ones and higher level ones.
 * tweaked example template to be much more collapsed by default.
 * enhanced links template to use the re-type-label, and rs-attribute-description directives.
* Mediatypes
 * make Links always list its attributes when describe (never shallow)
 * refactored media_type_collection to store a member_attribute (instead of a member_type), and report it in describe much like attributor collections do.

Signed-off-by: Josep M. Blanquer <blanquer@rightscale.com>