Add arange

Addresses #158. This pr adds np.arange(start, stop, step) functionality to spartan.arange() and does not introduce any API conflicts. This also adds more testing in `test_builtins.py.` In addition, this pr fixes a bug introduced in #159 (should be `return`, not `pass` on line 341).
Please split this test to at least two tests. One is the test with shape and the other is the test with stop.