Added zsh completion using bashcompinit

This is in reference to [issue 5](https://github.com/aws/aws-cli/issues/5). Since it is not the zsh-way to do auto complete I totally understand if you don't pull. OTOH, I've been using it for a couple of days and other zsh users might find it useful too. Please note, that I took a shell function from the zsh source to work around a bug that was fixed there 2 days ago. I don't think that it violates their [license](https://github.com/zsh-users/zsh/blob/master/LICENCE).

- Added snippet to make auto completion work in zsh using
      the bash compatibility mode
    - overwrote _bash_complete() to work around a bug that got
      fixed in
        [commit on sf](http://zsh.cvs.sourceforge.net/viewvc/zsh/zsh/Completion/bashcompinit?r1=1.5&r2=1.6)
      or
        [commit on github](https://github.com/zsh-users/zsh/commit/edab1d3dbe61da7efe5f1ac0e40444b2ec9b9570)
    - Tested in zsh 5.0.2, zsh 4.3.11 (i386-apple-darwin12.0) and zsh
      4.3.17 (x86_64-unknown-linux-gnu)
    - Added short description to README
    - Note: a proper compinit zsh function will follow later.