add IO functions
backport 3.x IO Zero functions
Generalize usage of IO[Zero]
bump v2.8
Add IO >>, publish v2.9
IO.inject for any IO
Merge branch '2.x'

* 2.x:
  IO.inject for any IO
  Add IO >>, publish v2.9
  bump v2.8
  Generalize usage of IO[Zero]
  backport 3.x IO Zero functions
  add IO functions

Conflicts:
	project/Build.scala
	src/main/scala/IO.scala