Partials caching

With this PR we pre-cache the directives' HTML partials in prod mode. This has several advantages:

* Performance - no per-partial Ajax requests
* The js build (build.js) is more self-contained - no external resources for partials

Note that generating Angular code for pre-caching of partials is a common practice. See https://github.com/karlgoldstein/grunt-html2js for example.

Also, it's interesting to note that we could do similarly for the locales files (en.json & friends). For that we'd use a custom `localeCache`.

Please review.