Fix bug where nicks could be missing the first char(s) in the nicklist

When a nick contains a PREFIX symbol that isn't at the beginning of
the nick, the _kiwi.model.Member#stripPrefix function would still
count it as a part of the prefix and remove an unrelated character
from the front of the nick.
Merge pull request #402 from M2Ys4U/nick_trunc_fix

 Fix bug where nicks could be missing the first char(s) in the nicklist