Added constructor analogs of all the `runXXXP` functions

This fixes #72.  I went beyond the original request to include `readerP`,
`writerP`, `stateP`, and `rwsP` "smart constructors".
Fixed documentation for `errorP` and `runErrorP`
Removed `foldl` dependency

I decided to remove the `foldl` dependency for a few reasons:

* Writing custom folds required importing `Control.Foldl`

* It was an extra dependency

* `foldl` is probably going to be unstable in the near future

I made sure all the folds in `Pipes.Prelude` can still accept the fields of
a `Fold` or `FoldM` type.

It is more likely that I will make `foldl` depend on `pipes` to provide
`Fold` integration for `pipes`.
Added note to `Pipes.Prelude` documentation

This explains that `stdin` and `stdout` remove and add newlines so that users
are not confused when they transition to `Text` and `ByteString` utilities.  See
issue #71.