Expose packing and unpacking primitives

[nh2]

I had to write some Packable and Unpackable instances for my own data structure and for this found the primitives in the package very useful. Unfortunately, they are not exposed, so that I had to copy-pasted them into my project.

This pull request exports:

• fromString, fromArray, fromPair, fromMap from Data.MessagePack.Unpack
• parseString, parseArray, parsePair, parseMap, parseUint16, parseUint32, parseUint64, parseInt8, parseInt16, parseInt32, parseInt64 from Data.MessagePack.Pack

It also adds Haddocks to all exported functions.

[wmoss]

This seems reasonable to me, no reason not to expose functions if they might be of use to people. @tanakh, what do you think?

[nh2]

Any update on this? It would be awesome to have it on hackage.

[wmoss]

This still looks good to me. I can merge it into github, but I don't have the ability to put it on Hackage and it seems strange to have github master not track Hackage relatively closely. Sorry for the delay, I don't know where @tanakh, is.