should be able to look at a function in isolation and prove it won't
panic. if that's not possible, should cast safely.

use case:

Just configured and installed a node.

benefits:

1) reduces friction when setting up a new node

2) reveals useful details about the workings of the system. It's the
user's first encounter with her node's identity. The tour can build on
the user's knowledge.

```
ipfs (maybebtc-november) λ. ipfs init -f
initializing ipfs node at /Users/btc/.go-ipfs
generating key pair
peer identity: QmcRbn41Vc2CvbpLYfN36mAWusErKWvAAHbq92LPra2gFT
```

Fixes #258

https://github.com/jbenet/go-ipfs/issues/258

add context to DialPeer interface

@jbenet

Fix data races in #270

https://github.com/jbenet/go-ipfs/issues/270#issuecomment-61826022

https://github.com/jbenet/go-ipfs/issues/270

Swarm + Handshake Listen Addresses

feat(tour) content

The check needed knowledge of the _listen_ addresses,
not just the interface addresses. Also, the handshake now
sends out all the addresses we accumulate about ourselves.
(this may be bad in the long run, but useful now to test)