Consistent encoding for shared secret

[callum-oakley]

This SDK disagrees with the shared secret that the other SDKs produce, because it encodes the master key to a latin1 string internally, and then concatenated with a utf8 string (the channel name) before being passed to the sha256 method which (probably) interprets strings as utf8. This is pretty insane, but it still "works" as long as you don't try to use two server libraries simultaneously, because then they will disagree about what the key should be.

Regardless, we should avoid all this encoding nonsense internally by just keeping everything in buffers once we've decoded from base64. After this change pusher-http-node agrees with the ruby and go SDKs at least. I'm going to check the others next.

edit: binary is a horribly confusing alias for latin1. yuck.