Add BROWSERID_CLIENT_SCHEME option

If you use `flask-browserid` on a server which has a reverse proxy in front of it (for example flask sitting behind nginx or apache), and the flask process is listening on `http` but the reverse proxy is listening on `https`, `flask-browserid` gets the assertion audience wrong resulting in failed Persona verification.

This adds an option to allow for setting the scheme (much like the domain name can be configured) to accommodate situations like this.

I've also added documentation for this and the existing `BROWSERID_CLIENT_DOMAIN` option.