Add a little more to the README about my low goals
Rename tree_node_t -> tree_t, for consistency with stack_t
The stack functions now takes a void pointer
Rename 'node' to 'data' for stack operations, now that the stack library is generic
Forgot to initialize some struct members when adding a string node to the AST
Valgrind gives better information with debug symbols
Now we can have multiple stacks
Added tree-freeing routine to make Valgrind happy

Will refactor all of the tree stuff out. It's getting a life of
its own, like the stack stuff.
WIP: Adding the interpreter