convert Input helper to use a Locator and classes. this allows adding of new Input types.
update tests
move a bunch of stuff around
- Move form helpers to their own dir

- Make 'field' helper a locator

- Make 'input' helper a locator (remove InputLocator)

- Add getField() method on form helpers to support Aura.Input array spec

- Select helper options() now recursively descends into array values as optgroups
change Input type Button to Generic and have the others extend that. No more $label and $label_attribs; instead, pass a 'label' pseudo-attribute.
Merge branch 'fields' into develop
finish merge from develop
remove unmerged original Field.php