Add missing `{-# LANGUAGE #-}` pragmas
Update `vector.cabal` to Cabal scheme v1.10

This effectively limits `vector` to GHC ≥7.0 (however, `vector` fails to
build with GHC 6.12 and older anyway).

This also moves the changelog from the description-field into a separate
`changelog` file (which is supported by Hackage2).
Add build-status icon and Hackage-link to README