iterators+multithreading on win32

[Николай Шафоростов]

Hi. We're getting crashes when using taglib from different threads (different files) on windows
all of them are in the crt functions called from std iterators (e.g. String's basic_string<ushort> iterator used in isLatin() methid, and also ByteVector's iterators).

The hypothesis is that std iterators are not thread-safe on win32.

What do you think if i convert the code to not use iterators (e.g. ifdefing the code)?