Merge pull request #55 from archblob/benchmarks

Pipes.Prelude benchmarks feedback.
Lowered cabal minimum version to 1.14
Updated tutorial
Renamed 'Iterable' to 'Enumerable'

The rationale behind this is not only because 'Enumerable' is a more
descriptive name, but also because if there is a higher-order equivalent to
'Traversable' then I would like to reserve 'Iterable' for that.
Renamed 'yieldAfter' to 'chain'

This follows a suggestion by James Deikun to make this name less verbose and
more descriptive of its purpose without sacrificing too much naming precision