Add support for updating (closing) an invoice
Fix to update invoice

Updating an invoice requires an unpaid invoice, which can only be
created by:
1) waiting for the period of a plan to elapse; or
2) manually creating a new invoice
In the test, we now manually create a new invoice.
added failure code to charges
Added property for cancel_at_period_end in Subscription
Added refundApplicationFee param to StripeCharge's Refund
Merge branch 'updating-invoice' of https://github.com/yeurch/stripe.net
Merge branch 'master' of https://github.com/kevjett/stripe.net
simplified test_data a little bit
Merge branch 'cancel_at_period_end' of https://github.com/hartez/stripe.net
Merge branch 'patch-1' of https://github.com/lennyrabin/stripe.net
updated readme, bumped version