Use go/build for guess cmd

Sample output:
```
$ glide guess
[INFO] 
# Detected project's dependencies.
import:
  - package: github.com/Masterminds/cookoo
  - package: github.com/Masterminds/cookoo/io
  - package: github.com/Masterminds/glide/cmd
  - package: github.com/codegangsta/cli
  - package: github.com/kylelemons/go-gypsy/yaml

```
Ref: #15