Require axs_testing explicitly instead a loop.

[Fixes #2]
Support Firefox usage.
Use Firefox compatible js attribute textContent
Adding jsdoc comments.
Fixes formatting for JS syntax
Add generated axs_testing with Firefox fixes
Add tests for browserUtils.
Require browserUtils in AuditRule js
Merge pull request #4 from ckundo/fix_dependency_js

Changes from ckundo:
- FireFox-compatible use of textContent and matchesSelector, plus test for the latter
- Improvement to test/index.html to pull in single generated axs_testing.js rather than looping over javascript files to include.