Version handshake

[cryptix]

addresses #166

[jbenet]

Let's do this instead:

message Handshake1 {
    // protocolVersion determines compatibility between peers
    optional string protocolVersion = 1; // semver

    // agentVersion is like a UserAgent string in browsers, or client version in bittorrent
    // includes the client name and client. e.g.   "go-ipfs/0.1.0"
    optional string agentVersion = 2; // semver 

    // we'll have more fields here later.
}

We dont need protobof to parse semver, it's a well established format, and we can parse it separately. (go-semver)

[jbenet]

Hey @cryptix, CR done. Major points:

• net/version -> net/handshake
• use go-semver and version strings
• simplify the select business (happy to do this, too, i've a pretty good sense of what i meant and may not have communicated it perfectly)

[jbenet]

@cryptix this is great! thanks!

[jbenet]

Ok, all set! Merged. Thanks!