Proof of concept for adding a foreign exchange link to the shop.

Added a function that returns a link for a foreign exchange site
and showed it on the Shop page. Also included a test to make sure
it works.
Modified the foreign exchange link to convert the right price.

Changed the forex site to Wolfram Alpha from XE, which allow us
to convert the right number of AUD with one click. Upated tests to
reflect this change.
Updated the foreign exchange link to use configured currency.
Added conversion links for recommended prices.

The paragraphs were getting a bit verbose so we shortened the link
text as well. Added an additional test to make sure the link is
working here too.
Fix bug in order total; add conversion link to order show.

The old order total method did not account for quantities. The
view test for order totals was not catching this, so we've
strengthened it. We also added a conversion link to the order
summary page and created an additional test to check that orders
with more than one item were generating the correct total.
Merge pull request #243 from lucymclaughlin/forex_link

Forex link