i18n

Internationalization.

## This cannot be used on live shops yet

__This is only released to Partners so far, so please keep this quiet until a full launch.__
- [Subscribe to the Partners blog email list](http://www.shopify.com/partners/blog/subscribe-to-our-blog) if you haven't already for other pre-announcements
- [Internationalization docs](https://s3.amazonaws.com/shopify-perf/i18n-beta/ShopifyThemeInternationalizationDocumentation.pdf)

#### i18n Changes
- Converted all hard-coded text to strings
  - Format is `section.group.string`
  - e.g. `customer.addresses.city`
  - Strings must follow the three-tiered format
- Added `en.default.json` and `fr-CA.json` string files
  - If you're submitting your theme to the theme store, one default language will be required
  - Adding additional languages will give you a competitive advantage in the theme store, but are not required
- Changed `ajaxify.js` to a liquid file so it can use strings

#### Gift Card Templates
- A new requirement for themes, the gift card template is now included along with separate stylesheet

#### Minor Changes
- Reverted to using 1em in inputs so buttons/inputs don't shrink
- Minor addition to theme setting defaults so social media icons show
- Minor edits of comments
- Check if unset variables are `blank`, rather than falsey
- BEM for more consistent styles

@jonasll @Shopify/fed