Clone a specific branch

Hi,

I hope I'm not too late considering your message about the debian freeze in #87. This pull request fixes the issue #51, and if I can attach commits to a new issue without creating a new pull request, I'm sorry but I don't know how.

This pull request:
- takes commits merged in #87
- adds a commit to actually allow to clone a specific branch
- is rebased against the current master

I have tested it successfully with the following commands:
- `./vcsh clone git@github.com:Dridi/dotfiles.git` (gives me the useless master branch)
- `./vcsh clone -b vim git@github.com:Dridi/dotfiles.git` (not what I want, but works as expected)
- `./vcsh clone -b vim git@github.com:Dridi/dotfiles.git vim` :+1: 

I'll try to be very responsive regarding the freeze.

Cheers,
Dridi