Fix UnsynchTextFrame byte format

- change encodedBytes.Reader's ReadNullTermString behavior to strip off
  null byte(s)
- add WriteNullTermString function to encodedBytes.Writer
- include NullLength information in encodingbytes.EncodingMap
- increase size of UnsynchTextFrame by the null length of the encoding
- automatically null-terminate description string in
  UnsynchTextFrame.Bytes()