Implemented the preg_replace_callback() changes done in #63 in a PHP <5.3 compatible way

As discussed, I replaced the 3 function calls to anonymous functions by slightly modifying the functions themselves and then calling them by name. This should make Tweetnest work again on installations running on PHP 5.2 or lower. I don't think it is neccessary at this point to require 5.3+, as this was the only change that threw an error for me.

Note: I only tested this a few times, but the changes are fairly minor, so I don't see a reason this should affect anything other than the tweet parsing during import.