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cc @whyrusleeping

If the datastore has a value for the key, we already have
the block. We should not write it again. This will make redundant
writes much faster.

At the moment, a datastore.Has on leveldb is a GetBackedHas.
Track https://github.com/jbenet/go-datastore/issues/6

the u.Hash error can be safely ignored (panic) because multihash
only fails from the selection of hash function. If the fn + length
are valid, it won't error.

cc @whyrusleeping

(perhaps should be logged instead)

examples:

    http://golang.org/pkg/container/heap/#Interface

    http://golang.org/pkg/net/#Interface

    http://golang.org/pkg/sort/#Interface

Move go-ipfs/bitswap package to go-ipfs/exchange/bitswap

* Delineates the difference between the generic exchange interface and
  implementations (eg. BitSwap protocol)

  Thus, the bitswap protocol can be refined without having to overthink
  how future exchanges will work. Aspects common to BitSwap and other
  exchanges can be extracted out to the exchange package in piecemeal.

  Future exchange implementations can be placed in sibling packages next
  to exchange/bitswap. (eg. exchange/multilateral)

* to wrap datastore for ease of use
* to pass a non-responsive bitswap mock rather than performing nil
* checks internally

dependencies are vendored into Godeps/_workspace and commit versions are
recorded in Godeps.json

update datastore to e89f0511
update go.crypto