Add JsonAccessor

``` php
<?php
namespace Webnium\JsonPointer;

$accessor = new JsonAccessor(new Parser);

echo $accessor->get('/path/to/value', '{"path": {"to": {"value": 1}}}'), PHP_EOL;
echo $accessor->set('/path/to/value2', '{"path": {"to": {"value": 1}}}', 'fuga'), PHP_EOL;
```

Output:

```
1
{"path": {"to": {"value": 1, "value2": "fuga"}}}
```
