Fixed bug where certain bytes could be not rot13'd; Consolidated functions.

- Previously, byteReader.Read would return immediately if the underlying
  io.Reader returned an error, and would not rot13 the returned bytes. Now,
  even if there's an error, any returned bytes are still rot13'd before
  returning.
- Previously, rotByte and rotRune implemented per-character rotations, and
  Rot13 and Rot13Rune were just wrappers around these functions. Now Rot13
  and Rot13Rune implement this directly, and calls to rotByte and rotRune
  have been replaced by calls to Rot13 and Rot13Rune.