Fix versioned import paths.

Hi,

  I've seen on the mailing list that you added the gopkg.in [import path](http://gopkg.in/inconshreveable/log15.v1). There's however a small issue that the internal import paths are still pointing to github.

Specifically:

 - root.go -> import "github.com/inconshreveable/log15/term"
 - ext/error.go -> import "github.com/inconshreveable/log15"

  This leads to double "go get"-s. Importing from gopkg in will pull in the versioned paths, but since those reference the unversioned ones, those too will get pulled in. This will potentially cause two versions of the package to be present simultaneously, and even used half/half mixed.

  I've fixed the import paths to use gopkg.in internally too.

Cheers,
  Peter