- new targets 'gtest' and 'gtest_clean'
- updated target 'cleaner' to include what 'gtest_clean' does

- very simple test case 'test/sw_test.cc'

- Added main() to sw_test.cc
- added 'sw_test' executable to .gitignore

```
make gtest
./sw_test
```

> [==========] Running 1 test from 1 test case.
> [----------] Global test environment set-up.
> [----------] 1 test from IsTestTest
> [ RUN      ] IsTestTest.Failure
> [       OK ] IsTestTest.Failure (0 ms)
> [----------] 1 test from IsTestTest (0 ms total)
>
> [----------] Global test environment tear-down
> [==========] 1 test from 1 test case ran. (0 ms total)
> [  PASSED  ] 1 test.