Fallback URL handling revised to match Janrain's latest guidance 

Changes the way fallback urls are generated, and introduces a new option (:fallback_url) to control the behaviour, addressing issue \#19.

* By default, when :fallback_url not provided or set to :enable, the fallback url is now /openid/embed
* When :fallback_url == :legacy, the fallback url is as before e.g. /openid/v2/signin
* When :fallback_url == :disable, the fallback url is disabled (and users must have javascript working to sign in)
* In all cases, sending { html: { href: 'another url' } } will override the fallback url setting. 

Motivation:
* Old style fallback URLs cause "The token URL or xdReceiver has not been whitelisted" errors when used with non-legacy widgets. See https://support.janrain.com/entries/484814-The-token-URL-or-xdReceiver-has-not-been-whitelisted?page=1#post_23492268
* Legacy widgets have been made end of life since July 15th, 2013 

Note that there are a couple of other sanitary commits in this branch that do some minimal cleanup of the test environment so that I as able to test ok.