Add support for external modules.

This adds support for external module to be linked in to the Piksi firmware.  The external modules will be maintained separately, and must be checked out to the ext directory when building to be included.

Also added is a mechanism to register custom NMEA message dispatchers as will be needed for Bluetooth which will be one such external module.

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If the `nmea_dispatcher_register` function is the only thing you want Doxygen to find, you can mode the `addtogroup` directive just around that block.