From: v0.2.1
  To: v0.2.3
* Added mathematical operators from `std::ops` to `Nibble`

From: v0.1.8
  To: v0.2.1
* Added sval trait (src/traits.rs) -> A string value.
* Updated nibble to reflect this change.
* Minor incompatibility: Changed `hval` trait to return a `String` instead of `&'static str'`
* Moved matching converstions out of `Nibbles` into `Nibble`.
** `Nibbles` calls all matching conversions from `Nibble`
* Added `to_string` to `Nibble` -> Returns a binary string, MSB (left), LSB (right).
* Added `to_hex_string` to `Nibble` -> Returns one of '0x0' .. '0xf'
* Added `to_hex_char` to `Nibble` -> Returns 0..f
* Added compilimentary methods `new_from_` and `to_x_str` in `Nibbles`