Removed all prior code
Added public interface
Added documentation of the types and functions
Added "const" keywords in appropriate places
Renamed alloc/free functions

alloc function does also initialization, so it should be named "new". To
fit this scheme, the free function should be named "destroy"
Added copy function
Added function for selecting entries from a set
Added parameter for the predicate function
Fixup: Added missing "f" for "function" to copy function ptr
Added missing include
Fixup: added note on what the parameters of the predicate function are about
Fixup: We insert a pointer, not the value itself
Added util/attributes.h header file for utility attribute macros
Added CONTRIBUTING.md file
Added pizza-note
Merge pull request #11 from matthiasbeyer/define-interface

Define interface
Merge pull request #14 from matthiasbeyer/add-contributing_file

Added CONTRIBUTING.md file
Merge pull request #13 from matthiasbeyer/add-util_attributes

Header for attribute macros
Added avl.{c,h} files with first definition of the interface
Added implementation of avl_alloc(), which is just an alias for convenience