Improve SDK use with guest accounts

I was testing the sdk to do something silly with guest accounts but the client won't sync due to trying to query `/_matrix/client/r0/pushrules/`, which returns:

{"errcode":"M_GUEST_ACCESS_FORBIDDEN","error":"Guest access not allowed"}

Does this mean the SDK can't be used for guest accounts? is it a bug?
FWIW, I'd create a PR and add an example for #430

In a nutshell the way I am using the SDK is:

* `client = matrixcs.createClient(opts)`
* If necessary:
* `client.registerGuest()`
* `client = matrixcs.createClient(opts)`
* `client.startClient()`

Start by tracing the createClient, registerGuest, and startClient flow to the request for /_matrix/client/r0/pushrules/. Confirm how guest synchronization currently handles the M_GUEST_ACCESS_FORBIDDEN response, then verify that a guest client can start without the forbidden request and that the behavior is covered by an appropriate test or example.

Written by the indexing model from the issue text.