Switched from a global phpunit install to using composer to manage the phpUnit dependency and autoloading.

I added the phpUnit dependency to composer and used the autoloader generated by composer to include both the class files as phpUnit. 

Apart from describing your library, composer does a good job at autoloading and dependency management as well. PhpUnit is added as dependency in require_dev, this means that people that just use the library won't need it, only developers of the library itself.

To install dependencies and generate the autoloader do `composer install`.

This pull request also includes changes from #17.
This pull requests also adds .idea to .gitignore, those files are generated by phpStorm IDE.
This pull requests also prevents a strict warning in your unit tests.