Remove obsolete tests
Comment on content of rejection object
Prevent same Fcommand running simultaneously in page

- this is possible because the Fcommand could start an async action and then
  call the onSuccess function some time in the future.  In the meantime,
  invoking the Fcommand again may cause strange things to happen and is
  impossible to cleanly support in all cases.  So, prevent it, and send
  feedback to the bg page that the second Fcommand invocation was aborted.

- note that the cache needs to be cleared before each test for
  ContentScript.js.  TODO:  that should be abstracted out
Fix typo
Update TODO

- remove implemented stuff
Use --debug (as well as --fg-debug)

- it's more intuitive
Append to head only if it exists

- otherwise, append to document