Switch context after send data in Data.Conduit.Network

The idea is described by @kazu-yamamoto in http://www.yesodweb.com/blog/2014/02/new-warp and already implemented in `warp`.
Merge pull request #151 from knsd/patch-1

Switch context after send data in Data.Conduit.Network
Version bump
Merge branch 'master' of github.com:snoyberg/conduit
Version bump
exceptions 0.6 #153
Add a monadic unfoldM, which makes sense for streaming

As a plain function unfoldM is partial, since it may never return the
final list; but in a conduit context, it makes perfect sense.
Add a test for unfoldM