Introduce Controllers

[birkett]

This PR introduces controllers. Code from core.php and pictsharemodel.php has been moved into the controllers (and AbstractController where it's shared between controllers).

This also adds:

• Configuration class, which contains some helper methods for working with the application configuration.
• FileSizeFormatter class, to format byte values to human readable file sizes.
• Custom exceptions, to help when reading the code.
• Unit tests (need more of these! - current coverage is around 4% by lines of code, this will improve as the controllers are split up)
• composer.json / composer.lock - helps install the development tools and code analysis tools (PHPUnit, PHP Mess Detector, PHP_CodeSniffer, php-cs-fixer and PHAN).

There should be no breaking changes in this PR. Using PHP 7.0 all over now, so no going back. As per normal with these PR's - a full test is needed before merging. Unit tests are going to help with this.

(branched from #61 - linking to #57)

Dev notes for the future: Classes/, Controllers/, Models/ will eventually move into src/ Tests eventually rename to tests Web assets and servable content move into public/

[geek-at]

have you confirmed that all existing features still work with this?