Add a bootstrap service

[Stebalien]

@florianlenz has written a bootstrap service (https://github.com/florianlenz/go-libp2p-bootstrap) but it needs a bit more review. Once that happens, we should incorporate it into the host itself so that services can call into the bootstrapper if they need peers.

My primary motivation is the DHT. Currently, if we have no peers, the DHT simply fails to put values. Ideally, it would call into the bootstrap service then try again.

Action items:

• Review the bootstrap service/interfaces.
• Figure out how to attach it to the Host.

[florianlenz]

ah yeah, it totally forgot to fix the problems mentioned by @whyrusleeping's review (sorry). I will do it till Monday morning.

[Stebalien]

Np. I just want this tracked somewhere so we don't lose it.