Creating session

[andersem]

Using the auto-generated code from #1

The problem with it was:

1. The generator didn't identify the correct return types
2. Subclasses of Transaction aren't generated properly, events don't match properly

Adjustments:

1. Removed unecessary APIs to make the code manageable
2. Comment out TransactionApi until later
3. Move AuthenticationAPI with needed models from auth to checkout

To test creating session, run Examples.Program with the arguments: accountId clientId clientSecret, e.g. T12345678 bacf4dce-e330-45db-b564-70296955db68 c9d0235a-c62e-4980-b95c-2b96b818ccec

Next step is to make the generic Transactions work.