Add a get_config() function to plotly.py.
Add a test for get_config()
Switch over how plotly.py gets config info to use get_config.
Add sign_in test with config info.
get_credentials now ignores user-supplied 'falsey' values
Remove unnecessary 'global' statements
Add get_config to the list of public functions for plotly.py
Merge pull request #134 from plotly/andrew-flexible-sign-in

Andrew flexible sign in