Adding an Echo function

It was a pity not to see what was executed.

So I added an **echo** function to display int values

File changes:
* corefn.cpp : Added, implementation of the echo function
* Makefile: Modified, corefn.cpp added to compilation process
* main.cpp: Modified, just adding *createCoreFunctions* function call
* example.txt: Modified, adding an echo for test

ToDo: If you like this push, perhaps llvm tutorial should be updated (not in this repo, so I can't do it, but I'll be glad to do it)

PS: thanks for the doc and source code, great starting point to write my own language