px:mediatype-detect is very slow for large filesets

This is an attempt to improve on that. A further optimization
could be to completely bypass the 'detect-from-namespace' parts
when 'in-memory' is empty and 'load-if-not-in-memory' is false.
px:mediatype-detect: make more compact

Changed giant p:choose elements into p:add-attribute with the mappings
between extension/namespace and mediatype in an inline XML document.
Merge pull request #22 from daisy-consortium/mediatype-utils

px:mediatype-detect is very slow for large filesets