Improving code styling following standards

Improves code styling allong for both `/sendgrid` and `/test` following [pep8](http://www.python.org/dev/peps/pep-0008/) and [pep257](http://www.python.org/dev/peps/pep-0257/). 

*ps.: ignored `D101 Docstring Missing` and also  `D401	First line should be in imperative mood` issues as it was not my intent to modify content.*

Also removed an unused variable at `sendgrid.message.Mail.add_bcc` method implementation.