Change url for repo in README

There is an exception on heroku when follow current url:

```
Host key verification failed.
       fatal: The remote end hung up unexpectedly
       Git error: command `git clone 'git@github.com:spree/spree-multi-domain.git' "/tmp/build_22565x83fshqj/vendor/bundle/ruby/1.9.1/cache/bundler/git/spree-multi-domain-71b8348f29960ac16301b990aa34f6ca5db70f04" --bare --no-hardlinks` in directory /tmp/build_22565x83fshqj has failed.
```
Associate payment_methods and stores.

This will allow a spree installation to specify which payment methods
are used in which stores.

If a store has no payment methods associated with it, all available
payment methods are considered valid.  If a store has at least one
payment associated with it, then only associated available payment
methods are valid.

Fixes #47
Admin interface for setting payment methods.

If multiple payment methods are available, this will allow a user to
specify which payment methods are available on certain stores.

Fixes #47