Use at_key.to_string() in AtClient put, get, delete

[JeremyTubongbanua]

What Needs To Be Done

• AtKey needs to become an abstract class
• Force developers and preexisting implementations to use the extended classes: PublicKey, SelfKey, and SharedKey
• Namespace support in to_string()
• AtKey.to_string() needs to be developed. See at_java for examples.
• Utilize to_string() in at_client.cpp put_ak, get_ak, and delete_ak.