Add an ssl_verification option (more below)

This can be:
- bool (True): always check
- bool (False): never check)
- str: (check certificate at specified path)
Allow users to pass this on sign-in
Insert new verification settings into requests.
version bump --> 1.4.3 (local ssl verification settings)
Add test to check that plotly_ssl_verification setting gets set.
Merge pull request #142 from plotly/set-certificate

Set certificate