Add m4a MIME type
Implement swappable routing tables

Changes:

    * Move routing table to ::tanzer::router

    * Move routing table entry to ::tanzer::router::entry

    * Refactor code to use ::tanzer::router and ::tanzer::router::entry
Further decouple routing logic from ::tanzer::session into ::tanzer::router
Declare leap year false by default
Redundant Xan is redundant Xan