Strip trailing slashes from server urls:

wd assumes the pathname of the server's url does not end in a slash (for instance, to generate the new session url the logic is pathname += '/session').  However, node's url format utility automatically adds a slash for empty pathnames, so for instance 'http://localhost' becomes 'http://localhost/'.  This commit modifies buildConfigUrl to guarantee that there is no trailing slash in the returned url object.