Using magit-run-git-async for stashing

This solves issue #1647 for git stash communicating with GPG. My guess
is that the asynchronous process is actually able to read the tty that
it needs to talk with gpg-agent.

I looked at the same code on the 'next' branch, and `magit-stash` has
been totally redone. I was unable to properly reproduce my issue on that
branch, but that's because it was not working in other, unrelated, ways.

That said, this is more of a hotfix since it is branched off 'master'.
Merge pull request #1648 from andschwa/stash-async

Using magit-run-git-async for stashing
CONTRIBUTE.md: concentrate on the feature freeze and the roadmap
README.md: remove tarball and marmalade instructions

Using a tarball is no longer a supported installation method.  We
will continue to provide a tarball for distributions but users who
do not use their distributions native package manager, are strongly
recommended to use Elpa instead.

We will continue to make new releases available on Marmalade, and
once 2.1.0 has been released the README will once more explain how
to do so.
README.md: the other package is VC, not Version Control

From `(emacs)Version Control`:

  The Emacs version control interface is called "VC".
README.md: fix an url