Add SendGrid API Support

[Akkarinage]

Adds support for using SendGrid to send emails instead of relying on flimsy email providers with basic SMTP auth.

Added the following config to config/application.php:

'SendGridAPIKey'	=> 'your.SG.api.key',	// SendGrid API Key for sending mail. (https://sendgrid.com/docs/Classroom/Send/How_Emails_Are_Sent/api_keys.html)
					// If this API Key is set, it will be used instead of the SMTP settings.

When not null, FluxCP will use SendGrid's API to send emails. If left as null, SMTP or mail() settings will be used.

Yes, this would be a slimmer PR if we used composer, but alas we do not.