change event callback signatures to put "context" params first, then d3 params.
- for input, the signature is  key,value,d,i,j  -- where d,i,j are d3 params.
- for other events, the signature is name,d,i,j.  `name` is the custom
  "subevent" (application-specific) key, e.g. for submit events you may have
  'save' and 'draft' or something. Typically this corresponds to the button
  name.

Note this change breaks compatibility particularly for reset, submit
handlers. input handlers should work as previously.

add 'del', 'refresh' events and buttons for typical form actions that
HTML form standards don't address.
add Form.del, Form.refresh to module.exports

add button().disabled(fn), button().visible(fn)
for conditional enabling/display

reorg and document code a bit

update example/index.html
- add del, refresh buttons
- add conditional enabling/display
version bump (0.3.1)
add onDel, onRefresh helper methods
version bump (0.3.2)