Add Stripe-Version header based on API supported by library

This is an alternate suggestion for #135.  I disagree with specifying the supported version in a config file in the calling app (as used in #147), as users will not necessarily know that they need to change this following an update, and at the end of the day it's this library that's compatible with a particular Stripe API version, not their calling app.

This change should allow a user to update Stripe.net without the needing to upgrade their account in their Stripe dashboard.  Unit test included to verify that specifying a lower version changes the response from Stripe.  Unable to verify that specifying a higher version than supported works, since my test accounts are on the latest version and you can't create one on an older version.

JSON.net has been included in the tests to work around a strange problem where ncrunch would not build the test project.