tap by specifying clone_url directly

this patch makes `brew tap` can clone repository not hosted by GitHub.

I know some formulas that can't be in public, but needed to be shared with others.
So, I wanted to tap a repository not hosted by GitHub.

## Usage

    $ brew tap user@host:path/to/repo.git
    $ brew tap git://host/path/to/repo.git
    $ brew tap http://host/path/to/repo.git

    $ brew untap repo