DList head/tail -> headOption/tailOption
restored/deprecated existing head/tail; fixed test idiocy
fixed version (duh)
Merge pull request #602 from tpolecat/dlist-fix

DList head/tail -> headOption/tailOption
call-by-need instead of call-by-name in Bind#ifM, Bind#ap
Merge pull request #603 from xuwei-k/call-by-need-bind-methods

call-by-need instead of call-by-name in Bind#ifM, Bind#ap
add Cofree.unfold
Merge remote-tracking branch 'xuwei-k/cofree-unfold' into scalaz-seven
Endo is InvariantFunctor

https://github.com/scalaz/scalaz/blob/v6.0.4/core/src/main/scala/scalaz/InvariantFunctor.scala#L23-L25
EndoTest