Remove trailing periods from NEWS items
Extend haskell-decl-scan.el to support PackageImports & SafeHaskell

This extends the `haskell-decl-scan-mode` parser to support the following two syntax extensions
for Haskell's import declarations:

- PackageImports
    http://www.haskell.org/ghc/docs/latest/html/users_guide/syntax-extns.html#package-import

- SafeHaskell
    http://www.haskell.org/ghc/docs/latest/html/users_guide/syntax-extns.html#safe-imports-ext
Add aliased `turn-{on,off}-haskell-doc` commands

This removes the irregularity, that `haskell-doc.el` didn't follow the
`turn-on-<module-name>` convention, but instead required
`turn-on-haskell-doc-mode`. The previous symbols are retained as
aliases for compatibility.
Clean up `:options` in `haskell-mode-hook' defcustom & add `:link`s

The various conditional `fboundp` statements are removed and replaced
by their evaluation result for Emacs 23/24. Moreover, two `:link` are
added to point to the relevant Info manual section and to the
docstring for `haskell-mode'.
Clean up docstrings for `haskell-mode' and `haskell-mode-hook'

The docstrings are made a bit more compact and point to the Info
manual for further details.
Pre-evaluate conditional `(fboundp 'help-buffer)' expression

This expression is always t for Emacs 23/24, so there's little
reason to have this for legacy support in there.