Added basic testing infrastructure.

"testcase" is a sample file from which test cases are run.

$ ./neo testcase
test case [1 + 2] *PASS*
test case [3 - 5] *PASS*
test case [6 * 4] *PASS*
test case [50 / 2] *PASS*
test case [100 % 6] *PASS*
test case [(100 + (2 * 8) - 9 / 3)] *PASS*
test case [(25 - (8 * 7) + 56)] expected [25] obtained [-87] *FAIL*
test case [((100 + 8 * (2 + 3)) + 60) / 20 + 90] expected [100] obtained [844] *FAIL*
test case [a = b = c = 10] *PASS*
test case ['Apple' + ' ' + 'iCloud'] *PASS*
test case [a = 'Orange'] *PASS*
test case [a + ' Fruit'] *PASS*
total test cases=12 passed=10 failed=2