Overhaul of the API and support for the SendGrid Events API

- .NET 4.5 is now the default implementation.
- Changed the base namespace to SendGrid from SendGridMail.
- Changed the SendGrid class to the Mail class.
- Added a static Events helper and the EventData class.
- Reorganized the class files and added missing documentation from the
Interaces to the Concrete classes.
- Created a new NuGet package that also contains the XML Documentation
Comments file.