Add 'src' directory

This small commit does a little more restructuring of the program in
preperation for some more additions. Right now, all that happens is
'main.c' is moved into 'src' and a new .mk file is added inside of the
src directory. Eventually, 'src' will also contain any other C files
used for the cReddit main executable.
Merge pull request #31 from DSMan195276/src-dir

Patch to add a 'src' directory.