Add methods for refresh token & delegated auth.

Deprecate old delegation methods in favor of new ones.
Now we have 3 methods to call delegated auth:
* Fetch a new JWT using a valid id_token.
* Fetch a new JWT using refresh token.
* Fetch a delegation token for another API.

This closes #38 issue when new delegation token info does not match JSON
structure for `A0Token`.
Check non-nil for `parameters`.
Only allow non nil params for fetchDelegationTokenWithParameters.
Merge pull request #40 from auth0/bugfix-delegation-callback

Fix issue with Delegated Auth response payload