Django-sendgrid-v5

Thanks liavkoren for your contribution!

https://github.com/sklarsa/django-sendgrid-v5/pull/45

Thanks blackfire70 for 39

Bump to 0.7.0 to clean up semantic versioning

1) Adds ASM and suppression group support
2) Strips brackets from content-id since they are added back by sendgrid's api

Now supports Sendgrid's sandbox mode in DEBUG mode (by default).

https://sendgrid.com/docs/Classroom/Send/v3_Mail_Send/sandbox_mode.html

Thanks to Fandekasp for the code review and feature request

Fix per issue 2