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.
Can you try to use the 5.3 array() syntax? Unfortunately PHP 5.3 is still the version of PHP that ships still with all Fedora and Debian based Linux builds and I'd hate syntactic sugar to be the reason to break that compatibility.