Added magic variables to suppress unused local variables

We come up against problems like this in our codebase:

```php
<?php
try {
	...
} catch( Exception $e ) {
	return false;
}
```

This throws an unused local variable warning. Unfortunately we're unable to suppress this, so I came up with the magic variables detection in the PHP_PMD_Rule_AbstractLocalVariable class. This works same as the superglobals, and now I defined the $_suppress variable to ignore the problem described above.

```php
<?php
try {
	...
} catch( Exception $_suppress ) {
	return false;
}
```