Revert "Comment out test that raises bug in Firefox"

This reverts commit bef29dd4a8c7b45e34c04807ce60e6e5d81d0197.
Revert "Be more specific with currency tests"

This reverts commit 1c8a0e42df4524f296bdc667465034c90f2a16a7.
Use Intl.js Polyfill as `Intl` mock in unit tests

Formatted output differs across implementations; e.g. IE11 adds bidi
chars: http://stackoverflow.com/questions/25574963/ies-tolocalestring-has-strange-characters-in-results

Therefore makes it difficult to unit test just these helpers. In order
to overcome these problems, we can simply **force** the use of the
Intl.js Polyfill as an `Intl` mock.
Merge pull request #50 from ericf/tests-mock-intl

Use Intl.js Polyfill as `Intl` mock in unit tests