[FIX] use token parser instead of regular expression

[phproberto]

I wanted to test thhe plugin so I tried to make an override of:

administrator/code/com_k2/models/item.php

The plugin wasn't able to find the original class with regular expressions. Best tool to parse PHP source files is:

http://www.php.net/manual/en/function.token-get-all.php

I replace the regular expression with token_get_all().

[juliopontes]

Thanks @phproberto