Add, test/{end,start}.js; Update, lib/index.js

test/{end,start}.js:
  tests showing how to use the new queue targets: `first' and `last'.

lib/index.js:
  1. added support for new queue targets: `first' and `last'.
    * `first' can be used to create an infinite loop, which might be desirable in the case of watchers: programs that watch other programs and alert peeps when those programs die.
    * `last' can be used to skip all following steps except the very last.