Allow interview settings to be passed instead of read from web.config

For issue https://github.com/HotDocsCorp/hotdocs-open-sdk/issues/3, I've added a new constructor to WorkSession to allow an InterviewSettings object to be passed in instead of always trying to read from web.config. 

I've also had to add a setter for PersistentEncryptionKey, as otherwise FinishInterview fails with an exception in GetPersistentEncryptionKey()