Fix bug that consumed more logs while draining than Logz.io's bulk limit

Fix #7 Return formatted message if format string

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.

Rewrite most of the handler, tests were introduced

Updated readme with the supported versions

Fix formatter not being used.

Add tests

Allow custom logzio fields to top level structure

added the listener url to the documentation