Work better with github urls

This is a retake of pr #145, but using a different approach. For
reference the original commit message from
7eabe9c296c324fc384f5ace1bf3eeeb67981928 is copied here:

For watever reason, when I clone my repositories I get a repository url
qualified with the ssh:// scheme, which Minilla doesn't seem to expect:

```
   url = ssh://git@github.com/lestrrat/Data-Localize.git
```

This creats all sorts of havoc in the META.json resources section:

```
   "resources" : {
      "bugtracker" : {
         "web" :
"https://ssh///git@github.com/lestrrat/Data-Localize/issues"
      },
      "homepage" :
"https://ssh///git@github.com/lestrrat/Data-Localize",
      "repository" : {
         "type" : "git",
         "url" :
"git://ssh///git@github.com/lestrrat/Data-Localize.git",
         "web" : "https://ssh///git@github.com/lestrrat/Data-Localize"
      }
   },
```

This change fixes this by checking for the ssh:// scheme when stripping
git@ username from github repository URLs
(as discussed on IRC) These translations only work with github anyway, and if we special-case github, these should be left out, since http repo URL isn't always available at git:// scheme in non-github repos.

While that might be a backward incompatible change, i'm sure nobody uses Minilla outside github git URLs, and even if there is, the generated URLs are wrong already.