Improve Button/Host definition/handling/testing. Slightly RESTify some URLs.

Buttons only have a single action now (no default 'pushed'), and for format
to specify them has been simplified.

Buttons and hosts can invoked with:
  <http_base>/button/<id>
  <http_base>/host/<id>

Instead of the now obsolete:
  <http_base>/button?id=<id>
  <http_base>/host?id=<id>