Format tokens.l

Corrected some intendation, for making a straight line.
Also added some more whitespace for better readability
Clean main.cpp

Removed an obsolete "std::" statement, because there's a "using namespace std;" in Line 5.
Merge pull request #19 from Columbus240/patch-1

Format tokens.l
Merge pull request #20 from Columbus240/patch-1

Clean main.cpp
Add "extern" statement

"extern" allows for declaring a function through ExternalLinkage. This
function can be defined in native C/C++ code.
Update Makefile and code to compile