Add support for changing currency in Settings

Stripe is now available in Canada so you are now able to choose to process in either American ("usd") and Canadian ("cad") currency.

At the moment, "usd" is hard-coded as the currency variable in several spots which causes errors if your account is set to "cad". These changes allow the currency to be set as one of the variables in PAYMENT_PLANS.
Merge pull request #19 from huxley/canadian-currency

Add support for changing currency in Settings
Default current to USD

This enables the enhancement of making currency a setting
instead of being hard-coded, backwards compatible.
1.2.1

Upgrade stripe library requirements
Update recording of disputed per deprectations in API

see https://stripe.com/docs/upgrades\#2012-11-07
1.2.2
Remove tag.js
Remove broken template tags
1.3
Set api_key once
Set the api version
Add new event types to webhook signals
Handle the transfer.updated webhook
Refactor models to be more in line with API changes
Update changelog
Update installation docs
2.0b1
Merge branch 'refs/heads/master' into class-based-views

Conflicts:
	payments/templates/payments/_cancel_form.html
	payments/templates/payments/_change_card_form.html
	payments/templates/payments/_subscribe_form.html