Allow to use user-defined collection of keys for key exclusion helper

Added optional argument for excludeKeys method. If you want to override default collection of keys, you may replace it with your own. For example:
```php
$helper->excludeKeys($data, array('another_key'));
```

If you want to add keys which will be excluded, you may want to merge your keys with keys provided by default:
```php
$excludedKeys = array_merge($helper->getExcludedKeys(), array('another_key'));
$helper->excludeKeys($data, $excludedKeys);
```
Add matcher configuration
Move key exluder to JsonHelper class
Add BejsonEqualMatcher
Add JsonHavePathMatcher
Remove outdated spec