Fixing import bug and adding addtl compatibility between bytes/strings

`cStringIO` is not a valid Python 3 module, so the redundant and erroneous import in TTransport.py has been removed. I've also added type checks to the `writeString()` and `readString()` functions in TBinaryProtocol.py so that they may be used in existing implementations that require `bytes` objects be read/written, not just unicode strings.