@author William Panting

This module is meant to describe the interface 
for modules implementing the Islandora viewer hooks.
It also creates an admin interface/DB entry to record settings regarding 
what theme function to use under what conditions.
It provides a way to detect wich viewer is selected through the admin interface.
This functionality may be extended in the future.
A source of possible bugs is the deactivation of viewers: 
A function should be created that viewers call on deactivation to remove any DB entry
regarding their module.

DECISION DIMENSIONS FOR CHOSING A VIEWER:
DSID (do not suggest supporting)
PID (do not suggest supporting)
SOLUTION_PACK_DEFAULT (would be hardcoded, or configurable in solution pack admin)
    Solution pack dependent.
    If the code falls through to this and it doesn't exist we should 
MIME_TYPE (for aggregate content models this would be a unique string. 
            Is there a convention for mime-types of distributed media?
            multipart/vnd.islandora.content_model) 
    implemented 
    this is the most basic way to make the decision
CONTENT_MODEL (can override MIME_TYPE)
    The first dev who needs this needs to implement it.
    This is the prefered way to make the decision in Islandora.
COLLECTION (can override CONTENT_MODEL)
    The first dev who needs this needs to implement it.

HOOK:
islandora_viewers(mimetype)
    ON NULL: return an array(mime_type=>theme function)
    ON MIMETYPE MATCH: return the theme function
    OTHERWISE: return nothing

DETECT VIEWER SETTINGS:
islandora_viewers.inc: islandora_viewers_get_viewer_theme($mime_type)

EXPECTED VIEWER/THEME INTERFACE:
theme(PID, DSID) @TODO: exameine; Xhould this be the URI of datastream?
    CONTENTS IN VIEWER

EXAMPLES:

VIEWERS:
https://github.com/willtp87/islandora_flexpaper_viewer

This approach is extendible to the object level: we can use it to overide an object level theme (display) such as 'Object Details'.
It is also extendible to metadata display, though the details of a well performing implementation need to be ironed out:
Maybe a relation on datstreams describing their schema. So the datastream would not need to be accessed more than once and not need to be passed around.