Expose RouterMethod to push LoginRoute + post-login redirect path

[SanzSeraph]

This was meant for the react-stormpath repository.

This is a proposed enhancement, not a bug.

In a situation where the server replies with a 401 (for example, if you are using express-stormpath to validate AccessTokens,) it would be very useful to have a built-in way to push the login path with the "?next" parameter set to a prescribed path.

this.props.router.pushLoginRoute('/mypreviouspath');

This can currently be achieved with this.props.router.push('/myloginpath?next=' + encodeURIComponent('/mypreviouspath')); which is a bit messy.