Add xd_node skeleton file

New struct 'xd_node' added. Also added skeleton method for finding
a node within an xd_doc 'xd_find'
xd_doc: Internal methods now static (protected)
xmldiver.h: Include stdint header
xd_node: Add start-tag offset var 'off'
xd_node: Add _xd_node_init method

Use for initializing a node internally and zeroing memory or setting
default values.
xd_node: Add _xd_node_free method