Add support for Twitter API v1.1 and new GUI options for API

In addition to adding support for Twitter API v1.1, this also includes two new options on the settings page for Consumer Key and Consumer Secret. The generated bearer token is stored in the database and used for all future authentication requests to prevent unnecessary token generation. Saving the settings page with an empty Consumer Key and Consumer Secret clears the stored bearer token. Thanks to @s3u for the initial work adding the certificate and codebird code. 

Fixes #29