Add msgpack serialization format

[byu]

It would be nice to include msgpack as another serialization. It would be a benefit to not have to maintain different rendering processes for json and msgpack in my app.

http://msgpack.org/

[nesquena]

That's a great idea actually, I am not opposed to adding that as another format. However, I know nothing about msgpack and I do not use it in my apps. If someone wants to write a patch to add a to_msgpack format here: https://github.com/nesquena/rabl/blob/master/lib/rabl/engine.rb#L51 I would much appreciate it.

[byu]

I think I understand the engine well enough to attempt to add to_msgpack. It would mostly be a clone of Engine#to_json and Engine#format_json.

Question. How to go about the msg pack dependency?

There is a hard dependency, but it wouldn't be so nice for people that don't have it, or want it, installed:
s.add_dependency 'msgpack', '~> 0.4.5'

I think a soft dependency would work out nicely, but with the following warnings:

• extra gem the user needs to be aware of in order to use.
• to_msgpack will just error out if msgpack gem isn't installed.

Which way would you like me to proceed?

[nesquena]

Soft dependency with a note in the readme about enabling msgpack support.

[nesquena]

Also please add corresponding riot tests if you can (check out existing for inspiration). Thanks, nice to have another format enabled for RABL

[nesquena]

I have merged this in. Thanks! 4462ea8