Initialise interpreter

Begin writing skeleton of the interpreter--currently just writes a prompt
and exits.
Initialise functions add, save and exit

Define the functions so that the program will compile
Add Makefile, change some prototypes

Create the Makefile necessary to build the project, change some #defines to
const int declarations for type safety.
Write skeleton of interpreter

Write the basic skeleton of the interpreter program, but the functions it
calls haven't been written at all yet.