Refactored and some new features

Hi Gerard,

this PR has grown quite big in the last two days, but I think it could be useful for you too. What has been done?

* Added a whole bunch of new unit tests
* Moved everything into it's own namespace `\Jobby`
* Code should now respect [PSR-2][] (but I haven't checked it with [PHPCS][])
* Refactored shared methods from `\Jobby\Jobby` and `\Jobby\BackgroundJob` into `\Jobby\Helper`
* The locking mechanism is now based on [flock][]
* New option `maxRuntime` added
* `.travis.yml` add (status of my fork: [![Build Status](https://travis-ci.org/michaelcontento/jobby.png?branch=refactored-and-new-features)](https://travis-ci.org/michaelcontento/jobby))
* `SwiftMailer` dependency updated from _v4.2.1_ to _v4.2.2_
* `Cron-Expression` dependency updated to _v1.0.0_
* The lockfile now contains the `PID` of the running process
* All errors within Jobby are written to the log _AND_ reported as mail

Any feedback or change requests are welcome :)

Best regards, Michael

  [flock]: http://php.net/flock
  [PSR-2]: https://github.com/php-fig/fig-standards/blob/master/accepted/PSR-2-coding-style-guide.md
  [PHPCS]: https://github.com/squizlabs/PHP_CodeSniffer