Added group, groupBy and span.
Merge branch 'master' of github.com:purescript/purescript-arrays
Implemented groupBy and span in ffi. Added group'.
Rewrote with accumulators to trigger tco.

Re: https://github.com/purescript/purescript-arrays/pull/5#issuecomment-42902783
Merge pull request #5 from joneshf/master

Added group, groupBy and span.