normalize exception name

This way it'll work better. It looks that this method can be even more simplified, but ... maybe some other time :)

Test cases of normalizeClassName:
```
$a = array(
    'Exception',
    'MoreException',
    'More_Exception_Foo',
    'Exception_Foo',

    'one/Exception',
    'one/MoreException',
    'one\More_Exception_Foo',
    'one/Exception_Foo',

    'one/two/Exception',
    'one/two/MoreException',
    'one\two/More_Exception_Foo',
    'one/two/Exception_Foo',
);
foreach($a as $s) {
    echo $api->normalizeClassName($s,'Exception')."\n";
}
```
output:
```
Exception
Exception_MoreException
Exception_More_Exception_Foo
Exception_Foo
one\Exception
one\Exception_MoreException
one\Exception_More_Exception_Foo
one\Exception_Foo
one\two\Exception
one\two\Exception_MoreException
one\two\Exception_More_Exception_Foo
one\two\Exception_Foo
```