Remove dependency on PHPUnit

[thewilkybarkid]

This has been raised before (#8), but I would like to see it revisited. PHPUnit assertions are currently used, but this involves pulling 17 (I think) other packages for just 6 assertions. We're not using PHPUnit in a project, so this is dramatically increasing the number of dependencies.

As PHPUnit doesn't provide its assertions in a standalone library, I think something else should be used, or they could just be replaced with internal ones (less convenient, but there's not that many).

[stof]

Feel free to send a PR refactoring the context to stop using the PHPUnit assertions.
but make sure it behaves the same (i.e. valid cases and rejected cases are the same), and make sure failure messages are good too.