Add hpack files for dynamic configs and set explicit constraints to help nix users

[noughtmare]

See yi-editor/yi-editor.github.com#21 for the nix issues

[noughtmare]

@ethercrow, are we removing all the cabal files from the repo or not? I believe you said it was confusing that .cabal files are in the .gitignore file and present in the repo, but I don't remember the conclusion.

[ethercrow]

I see three sane alternatives:

1. Don't use hpack at all
2. Keep only hpack files in the repo and have cabal files gitignored
3. Keep both hpack and cabal files in the repo and have Travis check that these are in sync with each other

[noughtmare]

I'm leaning more towards 1, because it is less confusing over all and also because cabal new-build is gaining traction and doesn't include hpack.