man.t: strip ansi escape codes

On FreeBSD the `ack --man` output read by man.t contains ANSI escape
codes, which breaks the test.  Update strip_special_chars to also
strip ANSI escape codes, fixing the test on FreeBSD.
Fix encoding issue with POD
Merge pull request #409 from frasertweedale/fix/man.t-strip-ansi

man.t: strip ansi escape codes
Add Fraser Tweedale to contributors
ignore Cabal sandbox directories by default

Cabal sandboxes are sandbox environments for Haskell packages,
containing compiled packages and a bunch of metadata that users of
ack are unlikely to want to search.  Therefore ignore .cabal-sandbox
directories by default.