Update README

I have fixed a few problems with the README examples:

1) There are some hidden "\n" strings (two characters) in the "Incompetent hackers trying to change the signature" serialization. I don't think they belong there.

2) A missing "=" at the end of the BASE64 serialization of the first empty macaroon. At least my .NET framework insists on it.

3) The signature of the first third party caveat example is wrong - its a copy/paste from a previous example with 3 caveats.