Deprecate encoder= calling parameter in all APIs

[lmctv]

The encoder= parameter in our API leads to some confusing bugs like #525; if we remove all its usages, we'll be able to remove the nacl.encodings module which just duplicates functionality already present in the standard library. The involved modules are:

• [ ] signing
• [ ] public
• [ ] secret
• [ ] hash
• [ ] pwhash
• [ ] encoding

[lmctv]

@reaperhulk @pyca/pynacl-core I'm thinking a possible way forward could be to postpone #523 and release 1.4. In this case, I'd just like to add some notes about the pending deprecation of the encoder parameter in the docs before the release.