Set default id attribute on form helper elements

Setting a default id on elements created by form helpers makes creating
input/tag pairs easier. For example:

    <%= label_tag :first_name %>
    <%= text_field_tag :first_name %>

results in:

    <label for="first_name">First name: </label>
    <input type="text" id="first_name" name="first_name" />

Radio buttons are handled a bit differently. The default id will be
generated from the name and value. If no value exists the name alone
will be used.