Setup project dependencies with Godep and -copy=true

Godep is moving away from the current format we're using to reference
dependencies. Instead, Godep is moving toward a workflow that vendors
all Go dependencies under the `Godeps/` directory.

Warnings have been added to the Godep source code when using the
-copy=false method:

  https://github.com/tools/godep/blob/master/go.go#L169-L177

Further discussion on the change:

  http://goo.gl/RpYs8e
Merge pull request #34 from OpenTreeMap/hmc/godep-round-2

Setup project dependencies with Godep and -copy=true