Support same name repository

Support same name repository like Bundler. (see also: http://bundler.io/v1.6/git.html)
We can `ghq get ruby` then clone `https://github.com/ruby/ruby` now.

(I'm sorry for new last line...)

---
Do I support `/ruby` ?

```diff
diff --git a/url.go b/url.go
index 3912537..9cbb802 100644
--- a/url.go
+++ b/url.go
@@ -29,8 +29,8 @@ func NewURL(ref string) (*url.URL, error) {
        }

        if !url.IsAbs() {
-               if !strings.Contains(url.Path, "/") {
-                       url.Path = url.Path + "/" + url.Path
+               if strings.Index(url.Path, "/") <= 0 {
+                       url.Path = url.Path + "/" + strings.Replace(url.Path, "/", "", 1)
                }
                url.Scheme = "https"
                url.Host = "github.com"
diff --git a/url_test.go b/url_test.go
index e99c46e..e97b6a2 100644
--- a/url_test.go
+++ b/url_test.go
@@ -34,6 +34,11 @@ func TestNewURL(t *testing.T) {
        Expect(sameNameRepository.String()).To(Equal("https://github.com/same-name-ghq/same-name-ghq"))
        Expect(sameNameRepository.Host).To(Equal("github.com"))
        Expect(err).To(BeNil())
+
+       sameNameRepositoryWithSlash, err := NewURL("/same-name-ghq")
+       Expect(sameNameRepositoryWithSlash.String()).To(Equal("https://github.com/same-name-ghq/same-name-ghq"))
+       Expect(sameNameRepositoryWithSlash.Host).To(Equal("github.com"))
+       Expect(err).To(BeNil())
 }

 func TestConvertGitHubURLHTTPToSSH(t *testing.T) {
```