Added basic multi-select field implementation.

Currently, it has a parameter for generating checkboxes but
does not respect it. It always generates a select field with
the multiple option set.

Additionally, we currently only support SelectableOptions
inbound, not tuples or simple object lists.
Changed selectFields to be defined in terms of calls to each other.

There is only one root function that actually does the
instantiating of SelectFieldHolder, the others are just calls
to overloads.
Drop some stray debug prints.
Add support for multiSelectField using tuples instead of SelectedOptions.

This is still only for a select box; no checkbox support yet.
Add support for multiSelectFields using basic objects instead of SelectableOptions.

Once again, this still only has support for select boxes;
checkboxes are coming next.
Add support for checkbox multiSelectFields with SelectableOptions.

Tuples and simple objects to follow.
Fix a couple of pluralization fails in the multi-select specs.
Add support for checkbox multiSelectFields with tuple options.
Add support for checkbox multiSelectFields with simple object values.