Rename control to form. They are essentially HTML5 forms so makes sense

Use the "form" name consistently, e.g. Yaks::Resource::Form,
Yaks::Mapper::Form, etc.

HALO still outputs under a "_controls" key.

Mappers that use "control :foo do" to create forms will see a warning.
Rough draft of an implementation of a Transit format
Add yaks-transit and yaks-sinatra to the top-level Gemfile
Add some convenience methods to Yaks::Resource::Form and ::Field

Access form values with [] or get the value hash with #values. Resolve
the value of a select list correctly.