Refactor Crypto Stuff

[DestructiveReasoning]

Based on the conclusions from #16, much of the current crypto implementation is obsolete.

Here's a quick list of changes that need to be made:

• [x] create_onion_packet() currently creates encrypted, layered JSON messages. Right now its behavior is different for establishment messages and onion messages, however data messages actually don't need layered JSON anymore (#16). Therefore, only the establishment part needs to stay, and perhaps be modified for compatibility.
• [x] A new function for encrypting onion messages (at the originator's end) will be necessary. This function should just encrypt with each symmetric key on the path sequentially.
• [x] The peel_layer function now should just simply decrypt packets it receives with its symmetric key
• [x] A function for decrypting response messages. I avoided doing this part while waiting for a stronger foundation of how our client was going to work. Now that we have it, I can implement this. Most likely just decrypting the response with each symmetric key sequentially.
• [x] Resolve #18.
• [x] Refactor class names. Currently some class names defined within the stealth/ directory, such as Originator and OnionNode, have misleading names. Once I'm refactoring this stuff I might as well rename them.
• [x] Update unit tests if necessary. The current unit tests in stealth/test.py are almost certainly going to be obsolete after these changes are made, aside from possibly the test_basic_rsa() test case. The other test cases should be refactored as well to preserve sanity and for good practice.

Following the resolution of this issue, we will be more or less ready to incorporate cryptography in onion_router.py and onion_client.py.

[DestructiveReasoning]

Starting progress in branch refactorCrypto-22.

[DestructiveReasoning]

Implemented symkey messages with OnionNode.create_symkey_msg(). Created Originator.create_onion() to make encrypted data messages. Similarly, created their respective decrypting methods and wrote unit tests to ensure that everything is working properly.

[DestructiveReasoning]

Refactored misnamed classes. Originator is now known as OriginatorSecurityEnforcer, OnionNode is now known as OnionNodeSecurityEnforcer.