Introduce vendoring

[JensRantil]

I had issues building locally in a preexisting GOPATH since it gocql was too old. How about introducing vendoring to both make migrate both forward and backwards compatible when it comes to building. My proposal is that we use dep for maintain the vendor directory.

[froderik]

Hmmm - I use this as a library in my code. If there is a vendor library in here I will have to flatten it somehow - right? It doesn't seem practical. Also given the huge amount of deps that migrate uses it will likely be clashes....