Issue #106: Use ES6 Promises instead of lavaca/util/Promise

Using [this Promise API](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Promise):
- Updated all functions which were directly constructing lavaca/util/Promise
  instances to construct Promise instances instead
- Updated all functions calling a `.then`, `.always`, `.success`, or `.error`
  method to only use ES6-compliant `.then` or `.catch` methods.
- Updated unit tests

I did skip updating lavaca/ui/Form and its unit tests... I'm somewhat sure it
will be removed in issue #121.

Side effect: ES6-ifying `mvc/View#render` into "sequential-looking" `.then`
chains resulted in removing two methods:
- `mvc/View#renderTemplate` because it boiled down to one statement:
  `template.render(model)`
- `mvc/View#bindRenderEvents` because it de-duplicated adding 'rendersuccess'
  and 'rendererror' onto the end of `render()` and `renderPageView()` at the
  expense of code flow readability imo. As an aside, I suspect we might be able
  to dedupe the entire `render()` and `renderPageView()` methods.