Added Doxyfile - doxygen configuration file
Added doc subdirs
Merge pull request #32 from matthiasbeyer/add-doxygen_config

Add doxygen config
Added avl.{c,h} files with first definition of the interface
Added implementation of avl_alloc(), which is just an alias for convenience
Added comperator func ptr arg to avl_add()

And adapted avl_alloc() appropriately.
Added infrastructure in avl_add() function
Fixup of the parameters of the find() and del() functions
Added helper function for finding elements
Added infrastructure implementation for avl_del() function
Added more comment to the helper function
Added implementation of the avl_find() function
Fixup for the initial import (argument types)
Added basic infrastructure implementation of the avl_unlink() function
Fix: We should name the data variable "data", not "d"

Suggested-by: Marcel Mueller <com2040@gmail.com>
Added documentation for the AVL tree node data type

Suggested-by: Marcel Mueller <com2040@gmail.com>
Added simple implementation for avl_destroy()
Fix: Return ptr to the deleted element on avl_del()
Fix: Added missing additional parameter for comparator function of avl_add
Remove the `cmp` argument for the `avl_add()` function