MangoPay V2 with card tokenization, authentication tokens storing etc

Added features:

• payment card tokenization support (see http://docs.mangopay.com/api-references/pay-in-tokenization/)
• authentication tokens caching improved
• errors handling improved: throwing exceptions on response error
• filtering params for fetch methods, including pagination support
• and more...

MangoPay V2

Here is the first release of the MangoPay gem that will work with the version 2 of the API

You can check out the changes on the first pull request.
For more details on how the gem works you can check out the wiki.