Set certificate

There have been issues with the ssl certificate settings. Though this should be a last resort, these changes allow users to (a) set verify = False or (b) specifically define the certificate they are expecting.

Open issues:
- [ ] do we need to do this for streaming?? the plotly.plotly.Stream object has no place for a 'verify', is that a different topic? i can't reproduce this error to check because my streaming cluster is down.