Use haskell-process-path-cabal everywhere

Does not change default behavior, only when these variables are
customized by the user.
Merge pull request #282 from bergey/cabal-path

Use haskell-process-path-cabal everywhere
Pick up import cycles as loading errors with fake line numbers

@hvr This has bugged me for a while.
Improve usefulness of file-relative-name in cycle code
Support GHC 7.8's message format for extension suggestions
Support 7.8 in extension completion
Add import suggestions based on haskell-docs
Add initial haskell-describe function
Fix inserting of type in haskell-describe
use notify instead of notifications-notify