This reverts commit d41ceabe82. Which I suspect
does not work because the thread is stateful, i.e. after os.fork the thread in
copy of the process thinks it has already been started, even though it is not
alive. Probably safest to just get a new thread instance if a thread is not alive.

Also, only use lazy logger string formatting evaluations

* Use non-marked format holders since we only support 2.7+
* Use format holders, only, no concat

If you pass an empty list, tuple, False, 0, whatever.. it will assume th
token is valid, which it isn't.

Currently the module raises generic Exception (very "don't do it")

Currently logging with:
```
logger.info('hello %s', 'world')
```
will send the string "hello %s" to logz.io instead of "hello world"

This commit fixes this situation.

Add tests

Allow custom logzio fields to top level structure