Some basic modifications to the Confirmation Dialog:
* Always attached to <body> regardless of view that called it.
* Always centered horizontally on screen.
* A black semi-transparent overlay is now displayed over everything, and under
the dialog.
* Various other style changes.
fixes#389
// FREEBIE
* Move install flow i18n logic to install_view.js (from options.js)
* Switch to using placeholders (instead of jQuery) for i18n messages with html.
* Switch to using moustache template instead of jQuery for i18n substitution.
// FREEBIE
Define a Whisper.View base class that automatically parses and renders
templates and attributes defined by the subclass. This saves us a good
number of lines of code as well as some marginal memory overhead, since
we are no longer saving per-instance copies of template strings.
