POST/GET Filters

I spent a bit of time last night and this morning working on this. Please try not to judge the Filter class too badly; PHP makes doing an unset at arbitrary depths kind of difficult. It would be so much simpler if you could do something like:

```php
$name = "my_array['test']['test2']";
unset($$name);
```

But to my knowledge it's not possible to do anything like that, so it has a really complicated, hard to understand bit of code using references.

I've included a few phpunit tests, you can make them work by just doing a
composer install inside the root of the project and then running phpunit tests/.

I've included usage instructions inside the README.md file.