Introductory pricing and other minor improvements

[devtobo]

This PR adds support for Introductory Pricing, introduced in iOS 11.2.

It also adds the following minor improvements:

• priceRaw field, similar to priceAsDecimal but sent as a string to avoid possible floating-point errors
• country code field (iOS only)
• transactionfinished event (iOS only), to be notified when a new renewable subscription transaction is processed

[coveralls]

Coverage remained the same at 86.111% when pulling 3300b8a on devtobo:introductory-pricing into d0765bb on AlexDisler:master.

[jonners99]

This would be lovely to get merged if it can be, support for iOS introductory pricing would be very nice.