Changelog for gray-code-0.3

- 0.3: use only total functions on `FiniteBits`, requires GHC >= 7.8 - 0.2.2: build on GHC 7.6, integrate tests with cabal build - 0.2.1: `toList'` function - 0.2: split `C.B.G.Bits` and `C.B.G.List` APIs - 0.1: initial Hackage release