made cache busting optional

[pthrasher]

Problem:

• Cache busting kills breakpoints set in the Chrome dev tools.
• Chrome dev tools already have an option to disable cache when developing.

Solution:

• Make the cache busting query string configurable.
• Ensure default behavior is no different than the current behavior.

To do this, I simple added a new settings var: GEARS_CACHE_BUST

[pthrasher]

@yumike Sorry for the initial PR, I hadn't realized it, but at the time didn't have all the code committed and pushed. I just updated it. Tests pass, plus I added one new test.