Binary Search Tree Implementation Started

The start of a binary search tree has been implemented. The code has not been
tested and there is still much to do. For items that need to be done, check for
TODO statements.

File(s):
   new file:   include/tree.h
   new file:   src/tree.c
   modified:   Makefile
   modified:   TODO.md
Makefile: updated for binary search tree implementation.
src/list.c: updated ll_rem(...) to supress uninitialized value warnings.
Merge branch 'linkedlist'
Merge branch 'tree'
Source code styling update.