Fix sing dec, trailing whitespace cleanup, .gitignore

- remove redundant check in `singDec`
- remove trailing whitespaces
- make sure all source files end with a newline
- add .gitignore

BTW. I hate the trash left by the pull requests in commit history. I'd suggest:

```
git remote add jstolarek https://github.com/jstolarek/singletons
git fetch jstolarek
git checkout -b jstolarek/fix-singDec fix-singDec
git rebase master
git checkout master
git merge --ff-only fix-singDec
git push origin master
git branch -D fix-singDec
```

(I hope I got this right)