Add mtl instances for MonadReader, MonadWriter and MonadState.
Fixed MonadWriter instance, added MonadError instance.

Fixed the implementation of the MonadWriter instance and moved liftCatchError
from Pipes.Lift to Pipes.Internal (reexported from Pipes.Lift) to implement the
MonadError instance.
Merged addition of unsafeHoist with mtl instances.
Merge pull request #59 from merijn/master

Add mtl instances for MonadReader, MonadWriter and MonadState.  Fixes #56
Merge pull request #60 from archblob/benchmarks

Add complete Pipes.Prelude benchmarks.
Added 'map' and 'filter'

... at the request of Nimish Telang
Merge branch 'master' of github.com:Gabriel439/Haskell-Pipes-Library
Tighted up documentation in Pipes.Prelude
Fixed 'foldM' to match the signature necessary for the `foldl` library
Added `length` fold
Changed 'debug' to be a 'Pipe'
Removed 'yieldIf'
Upgraded 'chain' to a 'Pipe' and removed 'debug'
Changed 'read' to be a 'Pipe'
Removed replicate and replicateM
Moved Consumer section above Pipes section of Pipes.Prelude
Changed `Producer'` type synonym to restrict downstream input
Added `foldl` support to `pipes`
Renamed `foldl` to `fold` and `scanl` to `scan`