Use SimpleCov instead of rcov for coverage testing
Move namespace handling from Sequel plugin into the core

While namespace handling is mostly useful for model forms, non-model
forms could also benefit from it.

It's common to use the same base values for the name and id
attributes, so add a :key option for that.  Also support a :key_id
option so that the id can vary even if the name stays the same,
useful for multiple checkboxes or radio buttons.
Support radioset and checkboxset Input types

This abstracts the code used to display associations as radio
buttons (many_to_one) or checkboxes (one_to_many), and moves it
into the Forme core, where it can be used by anyone.

radioset and checkboxset share the :options option parsing with
the select type, which simplifies a lot of the code.
Document supported types and options in the README