PHP 5.2 support and fancy CLI. YAAAY!

Finally, it all works.

The problems were:

- There were some `__DIR__`s, this is not available in PHP 5.2, so it all failed and threw an Exception for a non-existing content dir.
- `mime_content_type()` is not available in every environment.
- The definitions for content file mime types in `defaults.php` were missing, so it could not get that information and returned `null` instead of `text/plain`.

Also, there's finally the fancy CLI I promised to create. And it got fancier than I thought. You can also view the fancyness on Travis, because Travis now uses that script for testing instead of `phpunit`. :)
So, go and install it: `test/bin/kirbytest install` from the root of the repo.

This PR also fixes some other bugs and adds some sugar to mime type detection (Fileinfo and a return value of `false` instead of `null`).

Additionally, it adds PHP 5.5 as allowed failure, so Travis will not call the build failed just because PHP 5.5 failed. PHP 5.5 is Alpha anyway, so not officially supported by Kirby yet.

Last but not least, it adds a try catch around the first `site()` call in `testing_bootstrap.php` to catch exactly the errors I needed to know when finding out what was wrong here. :)

PS: Look at that bar below. Isn't that fancy? #passing #travis