Fix styles for empty orders list in admin

Fixes #2734
Fix email validation error I18n symbol lookup

And include %{value} for interpolation in validation messages.

The current code hardcodes the I18n key and improperly assigns the error
which leads to several problems:
* You cannot override the translation message for models/attributes
* You cannot pass your own I18n key as a symbol (eg. :invalid_email)
* You cannot use symbol interpolation

This fixes the above problems and is the proper solution for issue #2346.

This fix also applies to 1-2-stable and 1-3-stable.
For a fix against 1-1-stable see PR #2729.

Fixes #2730
Add simplecov for spec coverage reporting.

Fixes #2723
Modify the look of the coupon code.

This moves coupon code to be better aligned with the cart buttons.
This significantly improves the way things look in alternate
localizations.

Fixes #2720
Improve documentation about using gem from GitHub
Improve documentation about using gem from GitHub

Fixes #2705
Tidy up ProductDuplicator class, add tests + fix #2684
Correct content length check in base_helper_spec test for meta_data_tags