Tweak Pango modules to work with 0.13.x packages

This involves some extra annotations here and there. The Cabal bump is
because all the 0.13.x packages require configCompilerEx from Cabal 1.18
onwards and the ‘vty’ package also requires 1.18. Fixes #576.
Add release.nix for Hydra builds
Fix killring operations.

I don't know where the bug was but just using NonEmpty list type makes
sure we never hit it again.
Stick some lenses into Killring while here
s/Funtions/Functions/
Implement M-= (#581)
Implement M-\ (#581)
Move countWordsRegion somewhere better
Depend on binary 0.7 on >= GHC 7.8

While this isn’t needed (GHC 7.8 ships with binary 0.7, we indirectly
depend on GHC), it’s more correct and might just prevent future
headache.
Use Proxy in Yi.MiniBuffer
Define action* in Yi.Lexer.Alex using lambdas
Remove pureM from Yi.Core
Add docs to combinators in Yi.Keymap.Keys
Use Mode Binary instance
Use a <$> instead of pure a <*>
Use OverloadedStrings and IsString with Data.Rope
Use lens more in Yi.Buffer.Misc
Use convenience operators more and add docs in a few places
add irc notifications for travis