Bug 901943 - Remove legacy code for cross-environment support

Because this project no longer supports browser or XpcShell
environments, the code enabling cross-environment compatability may
safely be removed.

This includes:

- Re-naming the test environment's global `cross` utility object to
  `helper` to more accurately convey its intended use. (and simplifying
  the its `require` method)
- Removing the git ignore directive for the browser-specific `vendor/`
  directory.

r=lightsofapollo