Fix doc pointers about 'improve'

Point to the internal Control.Monad.Free.Church besides the
'kan-extensions' package.
Merge pull request #19 from klao/fix-doc

Fix doc pointers about 'improve'
fixed error in (>>=)/fmap law
Merge pull request #21 from Jesin/patch-1

fixed error in (>>=)/fmap law
Added missing instances
MonadFix
Fix (MonadFix)
Merge branch 'master' of github.com:ekmett/free
4.0
Fixed bifunctors dependency
retractAp
Add a default definition from `wrap` based on Nickolay Kudasov's wrapT
from issue #18.
CHANGELOG: wrap default signature
Merge pull request #18 from fizruk/master

Added wrapT
doc fix and import cleanup