fix interfaces so they work in a nicer go style

[whyrusleeping]

This PR makes the go-multiaddr interface match my own views on how interfaces in go should work, namely:

1. Constructors should always return a concrete type, not an interface.

• this means that no extra casting is required to expose extra capabilities of the underlying type. While its not a big deal for the multiaddr type, take a look at the go-datastore package for why this one matters.

And

2. Interface methods should never contain references to the type in question (or really any other types from within the same package)

• this ensures that the same interface imported from two differently vendored packages implement eachother

[whyrusleeping]

this isnt finished yet as the interface still references the Protocols type. The optimal fix (i think) for that is to move the protocols into their own package. (what? go gets better with increased modularity?)

[jbenet]

great, this LGTM. this will be a breaking change.

great observations @whyrusleeping

[whyrusleeping]

@jbenet thoughts on making a go-multiaddr-protocols repo? if we do that, and merge this PR after, we should solve a lot of the 'different import path' issues we've been having.