Added changed for webhooks

Latest version of webhooks callbacks adopted
Testcase for Bug for Offer
Fixed parsing of offers inside of subscriptions
- empty offer was encoded as [] instead of undefined/null/not present,
  now [] results in deletion of offer property
EventResource for simple transaction added

nextCaptureAt added to subscription
merge and fix tests
Add missing class
Replace old API URL
Added email attribute
Added order paramter to list
Added tests
Added tests
Revert "Added tests"

This reverts commit 6f9f916ea2133382de22673727b9fea73b1ae883.
Improve serialization of nulls in UrlEncoder and fix enum serialization in GsonAdapter
fix preauth create method
Fix integer overflow
Fix webhook email address
Fix line endings, fix error decoding
Throw an exception if the mandatory id parameter is null
Added constructors to Client and Payment to accept id as parameter
Merge pull request #13 from namero999/pull-requests

Added constructors to Client and Payment to accept id as parameter