This is done for two reasons:

1. There's no reason for it to be an instance method, since it does not
use the key

2. This gets us around the "hack" of creating a new AES instance with an
empty key, which breaks our validation of the key

* They are of the correct length (64 hex chars)
* They use only a secure source
* They pack and unpack correctly
* Converted the instance method to re-use the class method since key 
generation doesn't rely on any encryption state

The :base_64 format output of the random keys is very strange, because
you are base64 encoding the hex string, when the normal expectation is
you would base64 encode the raw binary. But I have left it as-is
for backwards compatibility.

They must be in the proper format and length: 64 character hex string
Raising an Argument Error

Also updated all examples with using a proper key

Finally, added a CHANGELOG with the breaking changes