Allow external command invocation with full path.

This PR permits the invocation of an external command with explicit 
fully-qualified paths, which is useful for troubleshooting.  For example,
I would like the following to work:
```bash
$ HOMEBREW_BREW_FILE=/usr/local/bin/brew /System/Library/Frameworks/Ruby.framework/Versions/2.0/usr/bin/ruby /usr/local/Library/brew.rb /full/path/to/external/brew-cmd.rb <verb>
```
This change also closes a small bug inasmuch as the above *does* partly-work:
the external command gets `require`d by what is currently the first conditional,
but an `undefined method` error is subsequently generated by `Homebrew.send`
at current line 106.