API Overhaul

* [x] #8 Extract DAGService and friends
* [x] Get a review on the last few commits
* [x] Get a final sign off.
* [x] Merge it!
* [ ] Fix all the other IPLD Node API issues all at once (annother PR)
* [ ] #9 Remove the Link structure, it's DagProtobuf specific
* [ ] #12 Remove the Size method
* [ ] #10 Remove the Copy method
* [ ] #11 Remove the Stat method
* [ ] #24 Merge the resolver interface into the Node interface.
* [ ] Update all other IPLD implementations to use the now fixed IPLD Node API.
* [ ] Update go-ipfs all at once.
* [ ] Fix ProtoNode to actually be a proper IPLD node and introduce a new FSNode interface for IPFS specific stuff.

I originally planned on doing this in stages but I'm now of the opinion that breaking everything all at once will be much less painful (it will still be very painful...).