Pull to Universal Branch - Added implementation for SAML Assertion grant

Added implementation for SAML Assertion grant and test cases. Also
updated the sample app to allow user to provide SAML Assertion that can
be consumed by the sample app. -
https://github.com/AzureAD/azure-activedirectory-library-for-objc/issues/205
It looks like this isn't being used in a mutable way?