PSR-2: leading underscores removed

* Property names SHOULD NOT be prefixed with a single underscore to indicate
  protected or private visibility.
* Method names SHOULD NOT be prefixed with a single underscore to indicate
  protected or private visibility.

https://github.com/php-fig/fig-standards/blob/master/accepted/PSR-2-coding-style-guide.md
moved Jobby and all tests into \Jobby namespace
add authors to composer.json
fix code example in README.md
add more unit tests for \Jobby\Jobby