[minor][client] Default URL to the browser window's origin.

If no URL is supplied in the client, just attempt to make a Primus
connection to the origin.

Useful if the webpage of the client is being served from the same Node.js server.
Also good for testing.

@3rd-Eden, pretty sure this is the "cleanest" way to do this, even though it sets the variable twice.
I'd rather not bulk move all the comments a couple tabs to the right, git blame and all.