Demo site changes

Always show form_opts and form_attr on the example pages.

Show the last album instead of the first, since the first album
is self-titled.

Don't show tracks association in the basic forms, since it doesn't
make logical sense, and the tags association shows all of the
same features.

Mention on the front page that the forms aren't submittable, and
that you should view the source to see the markup differences.
Add Forme.default_add_blank_prompt for setting default prompt used for :add_blank option

This is the simpliest way to handle the default prompt value.  It
can be made more granular (e.g. per form or per configuration) later
if there is a need.
Support :autofocus option for setting autofocus attribute, similar to :required and :disabled
Support Form :values option, for automatically setting value attributes for inputs from submitted params
Fix method signature for Form#inputs
Better documentation for Form#inputs
Allow input_defaults to work with symbol keys for inputs that use symbol types
Make subform always use the inputs_wrapper, even if :inputs is not given

Previously, subform only called the inputs_wrapper if :inputs was
provided, but that resulted in inconsistent behavior between:

 f.subform(:foo) do
   f.input(:a)
   f.input(:b)
 end

and:

 f.subform(:foo, :inputs=>[:a]) do
   f.input(:b)
 end

Ideally, these should result in the same output, the :inputs option
to subform and first argument to inputs just automatically handle
the input calls for you.

This makes subform pass a :subform option to Form#_inputs, because
I can't figure out how to make the Rails integration code work
without differentiating subform _inputs calls from non-subform
_inputs calls.
ol, div, and table inputs_wrappers now respect the :attr option
table inputs_wrapper now respects the :legend and :legend_attr options, using a caption
Refactor namespace/:key option handling

Make push/pop namespace create a new namespaces array instead of
mutating the namespaces array.

When creating Inputs, instead of manually setting :name, :value,
and :id options based on :key, just copy reference to the form's
current namespace.

Have the formatter look at the :key option and set the appropriate
attributes if they are not already set.  This is more similar to
how other attributes are set.
Remove get_wrappers method from Sequel plugin, no longer used
Remove Form#use_serializer and Serializer module

These were originally needed by the Sequel module when forme_input
could return an array.  However, that is no longer allowed,
forme_input must now always return an Input or Tag.
Fix coverage testing to it includes all lib files
Add Form.with_opts method to override opts for the given block

This takes quite a bit of refactoring because in the default case
(without templates), formatting is not done at the time of
Input creation.  So Inputs need to copy a reference to the Form
opts when they are created, and use that later when formatting.

Since the transformers for a form are no longer static, don't
set instance variables for them, just look them up in the opts.
The accessor methods for the transformers can go away as well.
The one exception is the serializer, which needs to remain
static.

Additionally, the Form#format and #serialize methods can be
removed as well, since nothing internally calls them, all
access goes through the transformer.
Form#inputs now accepts transformer options (e.g :wrapper) and automatically calls with_opts
Add tr inputs_wrapper and td wrapper, for horizontal layout of inputs inside a table
Support Form#inputs :nested_inputs_wraper option

This makes it possible to separate the inputs_wrapper used for the
the current inputs call, and the one used for inputs calls inside
the block.
Make table inputs_wrapper accept a :labels option and automatically set up th tags with the labels for each column
Add subform :grid option in Sequel plugin, for more compact editing of nested *_to_many associations

This allows you to edit multiple nested objects in a single table,
with one row per nested object and one column per field, with the
labels for each input as table headers.