accept SCP-like URL (git@github.com) for import command

* `url.Parse` doesn't handle SCP-like URL (e.g. git@github.com) properly.
```
fatal: invalid URL scheme name or missing '://' suffix
     error exit status 128
```
* now, `ghq import < FILE` clones repos into `.ghq/git@github.com:user/repo`.

* substituted `url.Parse` with `NewURL`, which accepts SCP-like URL as `doGet` currently implemented.