Support the add/update/delete recipient APIs

[kbussell]

My stab at #70. I've incorporated @smcoll 's feedback from #72, but retained the separate add and update functionality.

The UpdatedSigner object only existed to restrict the fields you could change on an update call, but it was simpler to just leave that out.

[smcoll]

@kbussell one drawback (which exists in my #72 as well) is that we're only allowing for addition/deletion of "signer"-type recipients. We aren't supporting addituon/deletion of other roles (agents/intermediaries/etc). That's all fine, but i wonder if we should consider what the function signature might look like if we did need to support that.

Would we be doing something like def delete_envelope_recipients(self, envelopeId, signerIds=None, agentIds=None, intermediaryIds=None...): or would we be sending a list of recipients of various types and then parsing it? If the latter, the function signature is just fine and we can deal with those cases later (that's my vote).

[zebuline]

Looks good to me.
As you said @smcoll, we can deal with those cases later, and change the signature of the method if it can be avoided (with compatibility and deprecation warning).
@kbussell there are some flake8 errors