Add default_form_class and allow form class overrides

This is especially useful with bootstrap forms where you might want to
switch to a `form-inline` layout even though the default is a
`form-horizontal`.

This deprecates SimpleForm.form_class in favor of
SimpleForm.default_form_class.

e.g.
config.default_form_class = "form-horizontal"
simple_form_for @user, html: { class: "form-inline" }
Updated changelog for default_form_class
Merge pull request #1109 from rmm5t/simple_form_for-config-overrides

Allow overriding of default configured form_class
Talk about the removal of the default simple_form class
We are defining the deprecated writer so we only need the reader