Added ability to use raw url for repo specification.
Added ability to use raw url for repo specification.

Now you can use a repo not hosted on github by specifying the full path to
the repo.
  - Supported urls match any of:
    - "git://*"
    - "https://*"
    - "http://*"
    - "ssh://*"
    - "git@github.com:*"
  - Anything not matching one of these is assumed to be the short form of a
    github repo: username/reponame.
  - Detecting local repos still uses the previous logic.

Copied the logic for url resolution used by antigen. I am not a lawyer, but
the licenses appear to be compatible.

yous/zgen,ixti/zgen,N4M3Z/zgen,outcoldman/zgen,unixorn/zgen,tarjoilija/zgen,m42e/zgen,forivall/zgen,sevanteri/zgen