Changed source directory

Now the headers and cityhash live in an inner libcuckoo/ directory,
rather than the lib/ directory. That way, programs in the examples/ and
testing/ directory can say "#include <libcuckoo/cuckoohash_map.hh>"
whether or not "make install" has been run.
Makefile cleanup
Enabled make check

Did away with the runtest.py script and made it so you could run the
tests using make check.