Replaced underscore as a JST for authentication attribute mapping

Rather than using underscore for attribute mapping, we now use basic
variable interpolation.

The syntax is of the form `{var}`.
For example:
Code:
    var template = '{firstName} {lastName}';
    var data = {'firstName': 'John', 'lastName': 'Doe'};
    var displayName = renderTemplate(template, data);
Result:
    John Doe

This also fixes a bug in the config API when the first value is a `{`.