Add test.c; define and doc BinaryTree_create().

data_structures/binary_tree/src/binary_tree.(c, h)
	-Define and document `BinaryTree_create()`.
	-Move `main()` to `test.c`.

data_structures/binary_tree/src/test.c
	-Add a module to serve as the main entry point for tests, so
	that the actual tree implementation can be compiled and linked
	into other executables.