Add XSLT functions for mediatype detection

 * Externalized the extension/namespace to mediatype maps to
   share the map in both XProc and XSLT implementations.
 * Added a new XSLT pf:mediatype-from-extension function
 * Added a new XSLT pf:mediatype-from-namespace function
 * Added XSPec tests for the mediatype functions.
Merge pull request #26 from daisy-consortium/mediatype-functions

Add XSLT functions for mediatype detection