Support for API v3.1, dry-run and per instance credentials

This release mostly adds support for Mailjet API version 3.1 and the Send API v3.1 in beta.

It also allows for dry-run calls, with the perform_api_call options.

It also also allows to manage API credentials local to a client instance rather than a global setting shared across all the instances (singleton configuration object).

The gem is available as always at Rubygems: https://rubygems.org/gems/mailjet