Use /:path instead of ?path=:path to get contents

Passing `path` in the query string does not seem to work anymore with the Github API (I'm admittedly guilty of not seeking to know why). The [API documentation](http://developer.github.com/v3/repos/contents/) says to use:

    GET /repos/:owner/:repo/contents/:path

which is what this patch does (and it works ;-)).