3.4/bugfix/add prefix useage to num helper

This fixes the issue of MB and MiB returning the same value
http://dev.kohanaframework.org/issues/4178
http://forum.kohanaframework.org/discussion/9445/textbytes-numbytes250mb-doesnt-return-250-mb-whyss-ko3.1

Someone please run the tests for me. I couldn't test them.